Low density microspheres

ABSTRACT

Low-density thermoplastic expandable microspheres are disclosed. Various low-density structures, in particular, sandwich panels, based on foam prepared from the low-density microspheres, are also disclosed. Process of preparing low-density polymeric microspheres, per se, and the corresponding low-density structures, based on the microsphere foam, are also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/525,656, filed on May 10, 2017, which is a National Phase of PCTPatent Application No. PCT/IL2015/051090 having International filingdate of Nov. 11, 2015, which claims the benefit of priority of UnitedKingdom Patent Application No. 1420055.4, filed on Nov. 11, 2014. Thecontents of the above applications are all incorporated by reference asif fully set forth herein in their entirety.

FIELD OF THE INVENTION

The present invention, in some embodiments thereof, relates tolow-density structures and, more particularly, but not exclusively, tolow-density polymeric microspheres, and methods of preparing same.

BACKGROUND OF THE INVENTION

The expandable microsphere is a material that can act as a blowing agentwhen mixed in a product and subsequently heated to cause expansionwithin the matrix. The expandable microspheres are off-white, can be 6to 40 micrometers in average diameter and have a density of 900 to 1400kg/m³. The expandable microspheres are used as a blowing agent inproducts like, e.g., puff ink automotive underbody coatings or injectionmolding of thermoplastics. Typically, the product must be heated at somepoint in the process for the expandable microspheres to expand.

U.S. Pat. No. 5,180,752 discloses a technique for drying and expandingthermoplastic microspheres. However, the limit of expandability istaught to be at a density of 0.015 g/cm³.

U.S. Pat. No. 6,864,297 describes process and properties for foam withdensity of 100 kg/m³. Sandwich panels are laminated compositescontaining two stiff skins and light density core located between andbonded to the skins. Skins are typically based on fiber reinforced layercomposites as well as ceramics or metals, or a combination thereof.Cores are based on light density foams including but not limited topolyurethane, styrene acrylonitrile (SAN) and polyvinyl chloride (PVC).Further, the core may be in a shape of light density honeycombs, balsawood, truss or corrugated structures.

Sandwich panels are widely used in airspace, navy, high speed groundtransportation, wind and gas turbine propellers, blast resistance, andengineering structures due to their light weight and their mechanicalperformance.

Sandwich panels operate as integrated skins-core structures and may befractured due to compression or shear core failures, wrinkling of skins,or debonding skins from core. Mechanical performance of core and skinscan be predicted and tested, and the mechanical performance of sandwichpanel can be designed by optimal way. The debonding of core and skinsmay go with unpredictable way due to existence of many uncertain factorssuch as possible existence of cracks and defects initiated atfabrication of a panel, and unpredictable behavior of those cracks anddefects under variations of temperature and climate conditions atutilization of the sandwich panel. Vibrations and impacts at fabricationand utilization may lead to unpredictable results and catastrophic bondfailures as well.

To prevent the delamination of skins from core several technologies weresuggested. One uses stiff Z pins inserted into core and slightlyprolonged into skins thickness. This technology is described in U.S.Pat. Nos. 7,731,046 and 8,272,188. This technology is consideredeffective at some degree by comparison to glue bonding. But thistechnology does not allow using the full potential of high strengthZ-pins due to a short length (a portion of millimeter) of Z pinsinserted into skins. At flatwise tensile testing, Z pins are pulled outfrom skins and never being tensile broken.

Another technology includes stitching skins to core using strong yarns.This technology is disclosed a number of patents, including U.S. Pat.Nos. 4,256,790, 4,828,206, 5,308,228, 6,431,837, 7,393,577, 6,740,381,and 5,589,243. Boeing, in its U.S. Pat. No. 6,187,411 notes that aprimary object using stitching sandwich technology is to receiveimproved flatwise tensile strength and substantially reduced damagepropagation from impact.

U.S. Pat. No. 8,288,447 discloses expandable, thermally curablecompositions useful as adhesives and structural reinforcement materials.

A major drawback of stitch-reinforced sandwich panels relates to thehole caused by the needle during stitching. As the space surrounding thestitching yarns cannot be left empty, resin is typically used to fill inthe stitch holes. Nevertheless, use of resin causes overweight of thesandwich composites, thereby limiting its use in many industries.

SUMMARY OF THE INVENTION

In a search for novel methodologies for fabricating low-densitystructures, the present inventors have surprisingly uncovered a methodof fabricating low-density polymeric microspheres, particularlythermoplastic polymeric microspheres, and more particularlythermoplastic expandable microspheres featuring the desired low-densityand surface chemistry, which exhibit exceptional properties. The presentinventors have fabricated foam prepared from the low densitymicrospheres, for use in various technologies, including, but notlimited to, sandwich panels.

According to one aspect of the present invention there is provided acomposition-of-matter comprising a powder of thermoplastic expandablepolymeric microspheres with a density below 15 kg/m³.

According to some embodiments of the invention, the expandablemicrospheres are characterized as having a uniform density. In someembodiments, the uniform density being characterized as having at least80% of the polymeric microspheres with densities that vary within arange of less than 20%. According to some embodiments of the invention,the uniform density is in the ranges of about 1 kg/m³ to about 25 kg/m³.

According to some embodiments of the invention, the expandablemicrospheres are characterized as having a uniform size. In someembodiments, the uniform size being characterized as having at least 80%of the polymeric microspheres with sizes that vary within a range ofless than 20%.

According to some embodiments of the invention, the thermoplasticexpandable microspheres are characterized as having glass transitiontemperature of up to about 250° C.

According to some embodiments of the invention, the thermoplasticexpandable polymeric microspheres are selected from the group consistingof: polyvinylchloride, polyacrylonitrile, polyvinylidene chloride,polyimide, and any combination and/or derivative, and/or copolymerthereof.

According to some embodiments of the invention, the powder of theexpandable polymeric microspheres is characterized as having a densityof below 1 kg/m³.

According to another aspect, there is provided process of preparing acomposition-of-matter comprising expandable microspheres, thepre-expandable microspheres being thermoplastic expandable polymericmicrospheres with a density below 15 kg/m³, the process comprising thefollowing steps sequentially: (a) subjecting the unexpanded microspheresto temperature that ranges from about 120° C. to about 180° C., for atime duration that ranges from about 2 minutes to about 10 minutes whilestirring; and (b) cooling said microspheres to room temperature at acooling rate of about 10° C. per min while stirring.

According to another aspect, there is provided a composition-of-mattercomprising foam comprising the polymeric microspheres, the polymericmicrospheres further being expanded and further being fused to eachother in at least a portion thereof.

According to some embodiments of the invention, the foam ischaracterized by a density below 25 kg/m³, wherein the density ischaracterized as being a uniform density, the uniform density beingcharacterized as having at least 90% of the foam with densities thatvary within a range of less than 15%.

According to some embodiments of the invention, the foam ischaracterized as having a density below 15 kg/m³. According to someembodiments of the invention, the foam is characterized as having adensity below 1 kg/m³.

According to some embodiments of the invention, thecomposition-of-matter further comprises one or more stochastic fiberfilaments. According to some embodiments, the stochastic fiber filamentsare reinforcing fiber filaments, the reinforcing fiber filaments beingaramid fiber filaments. According to some embodiments, the reinforcingfiber filaments are selected from the group consisting of: Kevlar,Nomex, carbon, glass, Poly(arylenebenzimidazole) (PABI),Poly(phenylenebenzobisoxazole) (PBO), Polybenzimidazole (PBI), and anycombination thereof.

According to some embodiments of the invention, the foam and/orpre-expanded microspheres further comprise a conductive additive, theconductive additive comprising a material selected from the groupconsisting of: carbon, a conductive polymer, conductive metal particle,a magnetic metal particle, metal alloys, ceramics, a composite materialand any mixture thereof. According to some embodiments, the material ischaracterized as having a size of at least one dimension thereof thatranges from about 1 nanometer to 1000 nanometers.

According to some embodiments, the carbon is in the form selected fromthe group consisting of: pristine carbon nanotubes, functionalizedcarbon nanotubes, multi walled carbon nanotubes, single walled carbonnanotubes, graphene, fullerene, carbon black, graphite, carbon fiber,and any combination thereof.

According to some embodiments, the conductive additive is in a form of acoating material on the foam and/or on the polymeric microspheres.According to some embodiments, the form of the coating material ischaracterized as having a form of porous networks. According to someembodiments, the form of the coating material is characterized as havinga layer with a thickness that ranges from about 2 nm to about 500 nm.According to some embodiments, the form of the coating material ischaracterized as having a layer with a thickness that ranges from about10% to about 200% of the thickness of the shells of the polymericmicrospheres. According to some embodiments of the invention, thecoating material is characterized as having weight that ranges fromabout 10 wt. % to about 200 wt. % of the weight of the polymericmicrospheres and/or polymeric foam.

According to another aspect, there is provided a sandwich panelcomprising: a first skin and a second skin; and foam. According to someembodiments the foam is a core, being located between the first andsecond skin. According to some embodiments, the core is stitched to thefirst skin and to the second skin using yarn beams. According to someembodiments, the yarn beams further comprise a resin. According to someembodiments, the foam fills a space surrounding the yarn beams.

According to some embodiments of the invention, the yarn is selectedfrom the group consisting of: carbon, glass, aramid, PBI, PABI, PBO,polyimide, polyamide, Poly(ethylene terephthalate) (PET), and anycombination thereof.

According to some embodiments of the invention, the sandwich panelfurther comprises a second foam of the invention, the second foamfilling the space surrounding the yarn beams. According to someembodiments, the second foam is characterized as having a density of atleast 30% higher than the density of the foam being located between thefirst and second skin.

According to some embodiments, the first skin and/or second skin furthercomprise a fibrous material and/or a resin. According to someembodiments, the yarn beams further comprise resin. According to someembodiments of the invention, the resin is selected from the groupconsisting of: phenolic, epoxy, polyetheretherketone (PEEK), polyimides,polyamides, bismaleimides, polyphenylene sulfide (PPS), polyphenyleneoxide (PPO), polysulphone (PSU), Polybutylene terephthalate (PBT).

According to some embodiments of the invention, the fibrous material isselected from the group consisting of: carbon, glass, aramid,polybenzimidazole (PBI), polyimide, polyamide, PET, and any combinationthereof.

According to some embodiments of the invention, the first skin and/orthe second skin are coated with a material selected from the groupconsisting of: a metal, a composite, a ceramic, a polymer, and anycombination thereof.

According to some embodiments of the invention, the foam is selectedfrom the group consisting of: PVC foam, polyurethane (PU) foam, styreneacrylonitrile (SAN) foam, polyethylene, polyimide foam, phenolic foam,and polymethacrylimide (Rohacell) foam.

According to another aspect, there is provided a method of fabricationof the sandwich panel of the invention, the method comprising the stepsof: stitching a first and a second skin to a core of foam using yarn,the foam having a density of below 25 kg/m³ and comprising expandablepolymeric microspheres; infiltrating thermoplastic expandablemicrospheres into the space surrounding the yarn; and heating theassembly to temperature that ranges from about 100° C. to about 285° C.to thereby expand and fuse the polymeric microspheres to the assembly.

According to some embodiments of the invention, the foam core isprepared from thermoplastic expandable microspheres being arrangeduniformly in volume unit by methods that include, without limitation,shaking, vibration, rotation, utilizing fluidized bed, and combinationthereof. According to some embodiments, the core of foam is reinforcedwith stochastic fiber filaments.

According to another aspect of the invention, there is provided ahoneycomb comprising the foam and/or expandable microspheres of theinvention. In some embodiments, the expandable microspheres at leastpartially fill either one or both sides, and part, or all cells of thehoneycomb.

According to another aspect, there is provided an article of manufacturecomprising the composition-of-matter, and/or foam as described herein.

According to some embodiments of the invention, the article ofmanufacture is in a form selected from the group consisting of: asandwich structure, an electromagnetic interference (EMI) shielding, ananti-radar-shielding, an antenna, a circuit board, noise damping,aircraft engine nacelle structure, airframe structure, aerospacestructure, heat-isolating structures, shock absorbent materials, blastresistance shielding, bullet and fragment resistance shielding, windpropellers, gas turbines, impact resistance structures, and marinestructures.

Unless otherwise defined, all technical and/or scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which the invention pertains. Although methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of embodiments of the invention, exemplarymethods and/or materials are described below. In case of conflict, thepatent specification, including definitions, will control. In addition,the materials, methods, and examples are illustrative only and are notintended to be necessarily limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the invention are herein described, by way ofexample only, with reference to the accompanying drawings. With specificreference now to the drawings in detail, it is stressed that theparticulars shown are by way of example and for purposes of illustrativediscussion of embodiments of the invention. In this regard, thedescription taken with the drawings makes apparent to those skilled inthe art how embodiments of the invention may be practiced.

In the drawings:

FIG. 1 is a scanning electron microscope (SEM) image illustrating theas-prepared powder of the pre-expanded microspheres, having an averagedensity of about 10 kg/m³; the bar line represents 200 microns.

FIG. 2 is a SEM image illustrating a fragment of a pre-expandedmicrosphere coated with multiwall carbon nanotubes; the bar linerepresents 600 nanometers.

FIG. 3 is a SEM image illustrating a fragment of a foam sample; the barline represents 18 microns.

FIG. 4 is a schematic illustration of the invented sandwich panel afterstitching, before filling with expandable microspheres.

FIG. 5 is a schematic illustration of the invented sandwich panel afterstitching and after filling with expandable microspheres.

FIG. 6 is a schematic illustration of a fragment of an ordinary usedcore of stitched sandwich panels.

FIG. 7 is a schematic illustration of a fragment of a core of theinvented stitched sandwich pane.

FIG. 8 is a dot graph representing data of the compression strength vs.the density of the foam samples.

DETAILED DESCRIPTION OF THE INVENTION

The present invention, in some embodiments thereof, relates tolow-density structures, and, more particularly, but not exclusively, tolow-density polymeric microspheres, and methods of preparing same.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not necessarily limited in itsapplication to the details set forth in the following description orexemplified by the Examples.

Low-Density Microspheres According to some embodiments, there isprovided a composition-of-matter comprising of polymeric microspheres.

As used herein and in the art, the term “polymer” refers to amacromolecule made of repeating monomer units.

The term “microsphere” as used herein is intended to encompassnanospheres, microspheres and also larger microsphere-like particles.Nanospheres generally have a diameter of 1000 nm or less. In someembodiments, the microsphere is about 10 to 2000 μm in diameter, ascharacterized by e.g., electron microscopy, such as scanning electronmicroscopy. In some embodiments, the microsphere is about 50 to 200 μmin diameter.

As known in the art, microspheres contain at least one radial pore. Thismeans that the pores extend from the central part of the microspheretowards the surface, e.g., parallel to the radii of the microsphere. Thepores may be tubular and interconnected. The radial pores provide themicrospheres with a level of mechanical strength.

As used herein, the terms “polymeric microsphere” and “microsphere”,used hereinthroughout interchangeably, refer to substantially sphericalpolymeric hollow particles.

In some embodiments, polymeric microspheres are thermoplastic expandablemicrospheres. The term “expandable” refers hereinthroughout to thecapability of the microspheres to increase its volumetric dimension.Expandable microspheres are spheres comprising a thermoplastic shellencapsulating a low boiling temperature liquid hydrocarbon. When heatedto a temperature high enough to soften the thermoplastic shell, theincreasing pressure of the hydrocarbon will cause the microsphere toexpand. The volume may increase by e.g., 60 to 80 times.

As known in the art, the term “thermoplastic microsphere” refers topolymeric microsphere, which includes any suitable material which isplastic or deformable, melts to a liquid when heated and freezes to abrittle, glassy state when cooled sufficiently. Once formed and cured, athermoplastic polymer is typically suitable for melting and re-molding.

Examples of thermoplastic polymers that may also be employed include,but are not limited to, polyvinylchloride, polyacrylonitrile,polyvinylidene chloride, polyimide, and any combination and/orderivative, and/or modification and/or copolymer thereof. Athermoplastic polymer may be functionalized to have additional benefitsthat include, but not limited to, water solubility or dispersability.

In some embodiments, the thermoplastic expandable microspheres of theinstant application are characterized as having glass transitiontemperature of e.g., up to about 80° C., up to about 100° C., up toabout 130° C., up to about 150° C., about 200° C., about 250° C., about300° C., about 400° C., about 450° C., about 500° C., about 550° C., orabout 600° C., or any value therebetweeen.

In exemplary embodiments, the thermoplastic expandable microspheres arecharacterized as having glass transition temperature of up to about 250°C. As used hereinthroughout and in the art, the term “glass transitiontemperature” is the temperature at which an amorphous material (or inamorphous regions within semicrystalline materials) enters a reversibletransition from a hard and relatively brittle state into a molten orrubber-like state.

In some embodiments, the thermoplastic expandable microspheres arepre-expanded polymeric microspheres. As used hereinthroughout, the term“pre-expanded” refers to the thermoplastic expandable microspheres,following a process as described hereinbelow, e.g., under “Process ofPreparing Pre-expanded Microspheres”, and as demonstrated in theExamples section that follows (e.g., Example 1 therein) aimed atreducing their density, and prior to their being used or processed ine.g., the molding equipment according to the invention.

In some embodiments, the pre-expanded polymeric microspheres are in aform of a powder. As used hereinthroughout and in the art, the term“powder” means finely dispersed solid particles.

In some embodiments, the pre-expanded polymeric microspheres arecharacterized as having a density of e.g., below 50 kg/m³, below 40kg/m³, below 30 kg/m³, below 25 kg/m³, below 20 kg/m³, below 15 kg/m³,below 10 kg/m³, below 5 kg/m³, below 4 kg/m³, below 3 kg/m³, below 2kg/m³, or below 1 kg/m³, including any value therebetween. In exemplaryembodiments, the density of a powder of the pre-expanded thermoplasticmicrospheres is about 6 kg/m³.

FIG. 1 shows a SEM image illustrating the as-prepared powder of thepre-expanded microspheres, having an average density of about 10 kg/m³.

In some embodiments, the pre-expanded polymeric microspheres (orexpandable microspheres) are expanded during processing which includes,but are not limited to, curing and heating. In the context of theseembodiments of the present invention Expancel™ and Sekisui™ microsphereshave processing temperature of up to about 200° C., and Matsumoto™microspheres have processing temperature of up to 300° C. Thesetemperatures render the microspheres and foam formed from thesemicrospheres suitable for applications that include, without limitation,aerospace and electronics processing and utilizations.

In some embodiments, the pre-expanded polymeric microspheres arecharacterized as having a uniform density. The term “uniform density” isused herein to mean substantially homogeneous density. In someembodiments the uniform density is characterized as having at least 90%of the pre-expanded polymeric microspheres with densities that varywithin a range of less than 10%. In some embodiments, the uniformdensity is characterized as having at least 90% of the pre-expandedpolymeric microspheres with densities that vary within a range of lessthan 20%. In some embodiments, the uniform density is characterized ashaving at least 90% of the pre-expanded polymeric microspheres withdensities that vary within a range of less than 30%. In someembodiments, the uniform density is characterized as having at least 80%of the pre-expanded polymeric microspheres with densities that varywithin a range of less than 30%. In some embodiments, the uniformdensity is characterized as having at least 80% of the pre-expandedpolymeric microspheres with densities that vary within a range of lessthan 20%. In some embodiments, the uniform density is characterized ashaving at least 80% of the pre-expanded polymeric microspheres withdensities that vary within a range of less than 10%.

In some embodiments, the uniform density of the pre-expanded polymericmicrospheres is about 30 kg/m³, about 25 kg/m³, about 20 kg/m³, about 10kg/m³, about 5 kg/m³, about 4 kg/m³, about 3 kg/m³, about 2 kg/m³, about1 kg/m³, or any value therebetween. In some embodiments, the uniformdensity of the pre-expanded polymeric microspheres is below 1 kg/m³.

In some embodiments, the uniform density of the pre-expandedmicrospheres is achieved any method known in the art of separatingmaterials according to their density, by, for example, for example,using a liquid column with varying density, or/and using centrifuge.

In some embodiment, the uniform density of pre-expanded microspheres isachieved by pre-separation by density of the unexpanded microspheres, byany method known in the art, as described hereinthroughout.

Uniform Size Microspheres

In some embodiments, the pre-expanded polymeric microspheres arecharacterized as having a uniform size. The term “uniform size” is usedherein to mean substantially homogeneous size. Size of pre-expandedmicrospheres means diameter of pre-expanded sphere.

In some embodiments the uniform size is characterized as having at least90% of the pre-expanded polymeric microspheres with sizes that varywithin a range of less than 10%. In some embodiments, the uniform sizesis characterized as having at least 90% of the pre-expanded polymericmicrospheres with sizes that vary within a range of less than 20%. Insome embodiments, the uniform size is characterized as having at least90% of the pre-expanded polymeric microspheres with sizes that varywithin a range of less than 30%. In some embodiments, the uniform sizeis characterized as having at least 80% of the pre-expanded polymericmicrospheres with sizes that vary within a range of less than 30%. Insome embodiments, the uniform size is characterized as having at least80% of the pre-expanded polymeric microspheres with sizes that varywithin a range of less than 20%. In some embodiments, the uniform sizeis characterized as having at least 80% of the pre-expanded polymericmicrospheres with sizes that vary within a range of less than 10%.

In some embodiments, the uniform size of the pre-expanded polymericmicrospheres is about 50 micron, about 60 micron, about 70 micron, about80 micron, about 90 micron, about 100 micron, about 120 micron, about140 micron, 160 micron, 180 micron, 200 micron, 220 micron, 240 micron,260 micron, including any value therebetween. In some embodiments, theuniform size of the pre-expanded polymeric microspheres is above 260micron.

In some embodiments, the uniform size of pre-expanded microspheres isachieved (or fabricated) by separating the desired microspheres by sizesusing e.g., filtration using calibrated size filters.

Foam Structures

According to an aspect of some embodiments of the present invention, theprocessed pre-expanded polymeric microspheres form a foam structure.

As used herein, the terms “microsphere foam”, “foam”, or “foamstructure”, which are used hereinthroughout interchangeably, refer to athree-dimensional porous material having a reticulated configuration incross section and which is pliable.

In some embodiments, the foam structure comprises polymeric microspheresbeing at least partially expanded. In some embodiments, the foamstructure comprises polymeric microspheres being at least partiallyfused to each other.

In some embodiments, the density of the foam is below 30 kg/m³, below 25kg/m³, below 20 kg/m³, below 10 kg/m³, below 5 kg/m³, below 4 kg/m³,below 3 kg/m³, below 2 kg/m³, or below 1 kg/m³.

In some embodiments, the foam is characterized as having a uniformdensity. In some embodiments the uniform density is characterized ashaving at least 90% of the foam with densities that vary within a rangeof less than 10%. In some embodiments, the uniform density ischaracterized as having at least 90% of the foam with densities thatvary within a range of less than 20%. In some embodiments, the uniformdensity is characterized as having at least 90% of the foam withdensities that vary within a range of less than 30%. In someembodiments, the uniform density is characterized as having at least 80%of the foam with densities that vary within a range of less than 30%. Insome embodiments, the uniform density is characterized as having atleast 80% of the foam with densities that vary within a range of lessthan 20%. In some embodiments, the uniform density is characterized ashaving at least 80% of the foam with densities that vary within a rangeof less than 10%.

In some embodiments, the uniform density foam is fabricated usingpre-expanded microspheres having a uniform density.

In some embodiments, the uniform density foam is fabricated usinguniform sized pre-expanded microspheres.

In some embodiments, the uniform density is e.g., about 50 kg/m³, about30 kg/m³, about 25 kg/m³, about 20 kg/m³, about 10 kg/m³, about 5 kg/m³,about 4 kg/m³, about 3 kg/m³, about 2 kg/m³, about 1 kg/m³, and anyvalue therebetween. In some embodiments, the uniform density of the foamis below 1 kg/m³.

In some embodiments, the foam comprises microspheres being fabricatedfollowing heating pre-expanded and/or expandable thermoplasticmicrospheres. In some embodiments, the foam comprises microspheres beingfused without a separate binder.

In some embodiments, the foam comprises microspheres of polyurathene(PU), styrene acrylonitrile (SAN), polyvinyl chloride (PVC),polyacrylonitrile (PAN), Polyvinylidene chloride (PVDC), polyethylene,polyimide phenolic, and polymethacrylimide (Rohacell) or any copolymerthereof.

In some embodiments, the foam comprises microspheres derived frompre-expanded or expandable microspheres which include, but not limitedto: Expancel™ microspheres, Sekisui™ microspheres, Matsumoto™microspheres, Henkel™ microspheres, or Kutsho™ microspheres.

In some embodiments, the microsphere foam is reinforced with stochasticfiber filaments. In some embodiments, the stochastic fiber filaments arearamid fiber filaments. In some embodiments, the stochastic fibers areat a ratio (wt. %) of, e.g., about 0.1%, about 2%, about 5%, about 10%,about 12%, about 14%, about 16%, about 18%, about 20%, about 25%, about30%, about 35%, about 50%, about 80%, or any value therebetween, of thefoam.

As used hereinthroughout, the term “reinforcing fiber filament” or anygrammatical diversions thereof, refers to a filamentary, band-shaped orstrip-shaped or webbing structure, which alone is intended to impart thenecessary rigidity and/or strength to the foam.

As known in the art, the mean fiber length of the reinforcing fiberfilament is greater than the critical fiber length in the foam matrix,or the combination, employed.

In some embodiments, the reinforcing fiber filaments include, but notlimited to, Kevlar, Nomex, carbon, glass, Poly(arylenebenzimidazole)(PABI), Poly(phenylenebenzobisoxazole) (PBO), Poly(benzimidazole) (PBI),and any copolymer or combination thereof. In some embodiments, themicrosphere foam is reinforced with Kevlar 29 stochastic fiberfilaments. In exemplary embodiments, the microsphere foam having adensity of 25 kg/m³ is reinforced with Kevlar 29 stochastic fiberfilaments.

FIG. 3 presents a SEM image illustrating a fragment of a foam sample.

Conductive Additives

According to some embodiments of the present invention, the foam of thepresent invention further comprises conductive additives.

In the context of conductive additive, the terms “polymericmicrospheres” or “microspheres” may refer to the pre-expendedmicrospheres or to the microspheres of the foam.

In some embodiments, the conductive additive is uniformly dispersedwithin the polymeric microspheres. In some embodiments, the conductiveadditive is incorporated onto the expandable polymeric microspheresshells. In some embodiments, the additive (e.g., carbon nanotube) islocated only on the microspheres' shells. In some embodiments,microspheres coated with conductive additives are processed to producethe foam with conductive additive.

In some embodiments, the conductive additives is infiltrated intocapillaries of foam, forming conductive passes within capillaries offoam.

Conductive additives usable in the context of these embodiments of thepresent invention include, but not limited to, any conductive additivesknown in the art, e.g., electro conductive polymers, carbon fibers,carbon fiber veils, ceramics electro conductive and ferromagneticnanoparticles.

In some embodiments, the term “conductive”, or grammatical diversionsthereof, refers to electrical conductivity of e.g., at least 10⁻⁷ S/m.In another embodiments, the term “conductive” refers to magneticconductivity.

In some embodiments, microspheres coated with conductive additives areuniformly distributed within foam. In some embodiments, microspherescoated with conductive additives are layered distributed within foam.

In some embodiments, microspheres coated with conductive additives(e.g., carbon fiber veils) form or are arranged in two dimensional (2D)patterns within the foam. In some embodiments, microspheres coated withconductive additives form or is arranged in three dimensional (3D)patterns within the foam. In some embodiments, the conductive additivesform 2D patterns within the foam. In some embodiments, the conductiveadditives form 3D patterns within the foam.

In some embodiments, microspheres coated with conductive additives areuniformly distributed within foam. In some embodiments, conductiveadditives are layered distributed within foam.

In some embodiments, the conductive additive comprises materials thatinclude, but are not limited to, carbon, a conductive polymer,conductive fibers, carbon fibers, carbon fiber veils, conductive metalparticle, a magnetic metal particle, metal alloys, ceramics, a compositematerial and any mixture thereof. In some embodiments, the conductiveadditives include, but are not limited to, gold, silver, copper, nickel,platinum, iron, cobalt and any combination thereof. In some embodiments,the materials are nanosized materials.

As used herein and in the art, the terms “nano”, “nanosized”,“nanomaterial” “nanotube”, “nanoparticle”, “nanoparticle composite”, orgrammatical diversions thereof, which are used herein interchangeably,refer to a property reflected by the particle size, and describe aparticle featuring a size of at least one dimension thereof (e.g.,diameter, length) that ranges from about 1 nanometer to 1000 nanometers.

In some embodiments, the size of the particle described hereinrepresents an average size of a plurality of nanoparticle composites ornanoparticles.

In some embodiments, the average size (e.g., diameter, length) rangesfrom about 1 nanometer to 500 nanometers. In some embodiments, theaverage size ranges from about 1 nanometer to about 300 nanometers. Insome embodiments, the average size ranges from about 1 nanometer toabout 200 nanometers. In some embodiments, the average size ranges fromabout 1 nanometer to about 100 nanometers. In some embodiments, theaverage size ranges from about 1 nanometer to 50 nanometers, and in someembodiments, it is lower than 35 nm.

In some embodiments, the average size is about 1 nm, about 2 nm, about 3nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9nm, about 10 nm, about 11 nm, about 12 nm, about 13 nm, about 14 nm,about 15 nm, about 16 nm, about 17 nm, about 18 nm, about 19 nm, about20 nm, about 21 nm, about 22 nm, about 23 nm, about 24 nm, about 25 nm,about 26 nm, about 27 nm, about 28 nm, about 29 nm, about 30 nm, about31 nm, about 32 nm, about 33 nm, about 34 nm, about 35 nm, about 36 nm,about 37 nm, about 38 nm, about 40 nm, about 42 nm, about 44 nm, about46 nm, about 48 nm, or about 50 nm, including any value therebetween.

The particle can be generally shaped as a sphere, a rod, a cylinder, aribbon, a sponge, and any other shape, or can be in a form of a clusterof any of these shapes, or can comprises a mixture of one or moreshapes.

In some embodiments, the composition-of-matter comprises a plurality ofnanoparticles, and at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%,95%, 98%, 99%, 99.9%, or all of the nanoparticles are nanoparticlecomposites as in shape and average size.

In some embodiments, the carbon may be, without limitation, carbonfibers, carbon black, single, double and multi walls and/or pristinecarbon nanotubes and/or, functionalized carbon nanotubes, carbonnanofibers, fullerene, graphene, graphite, carbon whiskers and anycombination thereof.

A conductive additive usable in the context of some embodiments of thepresent invention includes carbon nanotubes. In the context of theseembodiments of the present invention, carbon nanotubes gain particularlyattention for a wide range of electronics applications due to theiroutstanding electrical properties, nano sizes and light weight. Carbonnanotubes have electrical conductivity comparable to copper, and cancarry theoretically 1000 times more current density than e.g., cooper orsilver. As known in the art, carbon nanotubes further have outstandingmechanical properties, e.g., strength in the range of about 11-100 GPaand modulus of about 1 TPa.

In some embodiments, the density of carbon nanotubes is in the rangesof, e.g., about 1.5 gr/cc to about 2.5 gr/cc. In some embodiments, thedensity of carbon nanotubes is in the range of 1.6-2.2 gr/cc.

As detailed hereinbelow under: “Process of Coating microspheres withConductive Additive” according to an aspect of some embodiments of thepresent invention there is provided a process of preparing conductivefoam.

As used hereinthroughout, the term “conductive foam” refers to the foamof the present invention, comprising a conductive additive.

Typically, but not exclusively, the conductive additive is dispersed ina medium not harmful to the shells of the microspheres. In someembodiments, the microsphere's shell is coated with the conductiveadditive. In some embodiments, the conductive additive forms a networkstructure on the shell of the microspheres.

In some embodiments, the conductive additive forms a coating on thefoam.

In some embodiments, the conductive additive forms a coating withincapillaries of the foam. In some embodiments, the conductive additiveforms conductive passes within capillaries of the foam.

In some embodiments, the conductive additive forms a coating on thepolymeric microspheres. In some embodiments, the conductive additiveforms a coating having a porous network on the shell of themicrospheres. In some embodiments, the conductive additive is furtherdispersed in the spaces of the voids between the microspheres.

In some embodiments, the conductive additive is characterized as havinga layer with a thickness that ranges from about 1 nanometer to about5000 nanometers. In some embodiments, the conductive additive ischaracterized as having a layer with a thickness that ranges from about1 nanometer to about 1000 nanometers. In some embodiments, theconductive additive is characterized as having a layer with a thicknessthat ranges from about 2 nanometers to about 1000 nanometers. In someembodiments, the conductive additive is characterized as having a layerwith a thickness that ranges from about 2 nanometers to about 500nanometers.

In some embodiments, the conductive additive coating is formed on thinelastic shells of the microspheres. By “thin elastic shells” it is meantthat the thickness of shells of large microspheres are with diameter ofabout e.g., 50 micron, about 100 micron, about 200 micron, about 300micron, 500 micron and are less than e.g., about 400 nm, about 200 nm,about 100 nm, about 50 nm in size and are suitable to the sizes ofconductive additive coating.

In some embodiments, the conductive additive coating is characterized ashaving a layer with a thickness that ranges from about 50% to about1000% of the thickness of the shells of the polymeric microspheres. Insome embodiments, the conductive additive coating is characterized ashaving a layer with a thickness that ranges from about 20% to about1000% of the thickness of the shells of the polymeric microspheres. Insome embodiments, the conductive additive coating is characterized ashaving a layer with a thickness that ranges from about 10% to about1000% of the thickness of the shells of the polymeric microspheres. Insome embodiments, the conductive additive coating is characterized ashaving a layer with a thickness that ranges from about 5% to about 1000%of the thickness of the shells of the polymeric microspheres. In someembodiments, the conductive additive coating is characterized as havinga layer with a thickness that ranges from about 1% to about 1000% of thethickness of the shells of the polymeric microspheres. In someembodiments, the conductive additive coating is characterized as havinga layer with a thickness that ranges from about 0.5% to about 500% ofthe thickness of the shells of the polymeric microspheres. In someembodiments, the conductive additive coating is characterized as havinga layer with a thickness that ranges from about 5% to about 200% of thethickness of the shells of the polymeric microspheres. In someembodiments, the conductive additive coating is characterized as havinga layer with a thickness that ranges from about 10% to about 200% of thethickness of the shells of the polymeric microspheres. In someembodiments, at least one network of conductive additive wire coated theshells of the microspheres.

In some embodiments, the shells of the microspheres have nanosizedthickness.

In some embodiments, the conductive additive coats at least e.g., about10%, about 20%, about 30%, about 40%, about 50%, about 60%, about 70%,about 80%, about 90%, including any value therebetween, of the surfacesof foam voids.

In some embodiments, the conductive additive forms a uniform andhomogenously coating.

In some embodiments, the content of the conductive additive ontomicrospheres shells is e.g., at least 1%, at least 5%, at least 10%, atleast 20%, at least 25%, at least 50%, about 100%, about 200%, about500%, including any value therebetween, of the weight of the polymericmicrospheres.

In some embodiments, the impedance of conductive coating is fabricatedaccording to Direct Current (DC) requirements of an electronic device.In some embodiments, the impedance of conductive coating is fabricatedaccording to Alternating Current (AC) requirements.

“DC” refers to a type of electricity transmission and distribution bywhich electricity flows in one direction through the conductor, usuallyrelatively low voltage and high current. “AC” refers to a type ofelectrical current, the direction of which is reversed at regularintervals or cycles.

Further embodiments of this aspect of present embodiments are includedhereinbelow, under the “Coating microspheres with Conductive Additive”and under the “Fabrication of Powder of microspheres Coated with theConductive Additive”, and form an integral part of embodiments relatingto the conductive additive coating.

Multilayer Structures

As discussed hereinabove, currently known methodologies of stitching thecore panel involve the undesirable increased overweight of the corepanel, or lead to the formation of non-uniform density polymer foam.While conceiving the present invention, the present inventors havedevised and successfully prepared and practiced a method of fabricationof multilayer structure comprising uniformly low-density expandablemicrosphere foams.

In an embodiment of the present invention, the multilayer structure is asandwich panel comprising two skins stitched to a core of foam. In someembodiments the core is stitched to the first skin and to the secondskin. In some embodiments, the multilayer structure is a sandwich panelcomprising two skins stitched to foam core with yarn beams containingfiber filaments bonded together.

The terms “sandwich panel” and “sandwich stitched structure” are usedhereinthroughout interchangeably.

The terms “foam”, “microsphere foam” and “microsphere based foam” areused hereinthroughout interchangeably.

As used hereinthroughout, the terms “foam core”, or “core of the foam”,which are used herein interchangeably, refer to the foam being locatedbetween the first and the second skin.

In some embodiments, one or more of the skins in the sandwich panel arecoated with a material that includes, without limitation, a metal, acomposite, a ceramic, a polymer, and any combination thereof.

In some embodiments, the metal, ceramic, or the composite of the skincontain prefabricated drilled holes.

In some embodiments, at least one thin adhesive film is inserted betweenat least one the skin and at least one core.

In some embodiments, one or more of the skins of the sandwich panelfurther comprises a fibrous material. Exemplary fibrous materialsinclude, without limitation, carbon, glass, aramid, polybenzimidazole(PBI), polyimide, polyamide, PET, and any combination thereof.

In some embodiments, there is space surrounding the yarn beams insidethe foam core. In some embodiments, the space surrounding the yarn beamsinside the foam core is filled with heat expandable thermoplasticmicrospheres. In some embodiments, the heat expandable thermoplasticmicrospheres are substantially devoid of resin.

In the context of the present invention, the sandwich panel comprises atleast a first skin and a second skin and a core of the foam of thepresent invention.

Exemplary foams of the present invention include, but are not limitedto, PVC foam, polyurethane (PU) foam, styrene acrylonitrile (SAN) foam,polyethylene, polyimide foam, phenolic foam, and polymethacrylimide(Rohacell) foam.

In some embodiments, the core of the foam is stitched to the first skinand to the second skin using yarn beams. In some embodiments, at leastone of the yarn beams comprises yarns with elastic modulus e.g., atleast 10 GPa, at least 20 GPa, at least 30 GPa, at least 40 GPa, atleast 50 GPa, at least 60 GPa, at least 100 GPa, at least 200 GPa, atleast 300 GPa, at least 400 GPa, at least 500 GPa, at least 600 GPa, orat least 700 GPa.

In some embodiments, the yarn is at a ratio (wt. %) of e.g., about 5%,about 10%, about 20%, about 25%, about 30%, about 35%, about 40%, about45%, about 50%, about 55%, about 60%, about 65%, about 70%, about 75%,about 80%, about 85%, about 95%, or any value therebetween, of the foamcore.

In some embodiments, the yarn is prepreg of yarns. As used herein theterm “prepreg” is defined as fibrous material being pre-impregnated witha resin. Exemplary prepreg of yarns includes, without limitation, arenon-sticky prepreg of yarns, sticky prepreg of yarns, flexible prepregof yarns, partially crosslinking prepreg of yarns, partly curedthermoset resin prepreg of yarns. In some embodiments, the prepreg yarnsexhibit improved flexibility of the yarn. In some embodiments, theprepreg yarns comprise non-sticky resin to be processed through sewingmachine. In some embodiments, the prepreg yarns are coated withnon-sticky coating to be processed through sewing machine.

In some embodiments, at least one of the yarn beams comprises yarnswhich include, but not limited to, carbon, glass, aramid,poly(benzimidazole) PBI, Poly(arylenebenzimidazole) (PABI),Poly(phenylenebenzobisoxazole) (PBO), polyimide, polyamide,Poly(ethylene terephthalate) PET, or any combination thereof.

In some embodiments, at least one of the yarn beams comprises acombination of yarns containing thermoplastic yarns.

In some embodiments, at least one of the yarn beams comprises fiberfilaments. In some embodiments, the fiber filaments are thermoplasticfiber filaments.

In some embodiments, the yarn beams comprise yarns being attached to, orcomprising a resin. Exemplary resins include, without limitation, anuncured thermoset resin, cured thermoset resin, thermoplastic resin, anda liquid crystalline resin. In some embodiments, the resin of yarn beamscomprises a material that includes, but not limited to, phenolic, epoxy,polyetheretherketone (PEEK), polyimides, polyamides, bismaleimides,polyphenylene sulfide (PPS), polyphenylene oxide (PPO), polysulphone(PSU), Polybutylene terephthalate (PBT).

In some embodiments, at least one of the skins further comprises resin.In some embodiments, the resin of at least one of the skins is a curedthermoset resin. In some embodiments, the resin of at least one of theskins comprises a thermoplastic resin. In some embodiments, the resin ofat least one of the skins comprises a liquid crystalline resin.Exemplary resins include, but not limited to, adhesive films and theprotective sheets include, but not limited to, a phenolic, an epoxy,ampolyimide, a polyvinylchloride, a silicone, an amino, a polyesterresin, a PEEK, PEKK, polyamide, polyether, polyester, PP, PE, mono orbismaleimides, polyethylenetereftalate, polyamide, polyamideimide,polyetherimide, polyparaphenylenesulfide, ABS, polysylfone,polyacrylonitrile, polyvinylidene chloride, a copolymer, or a blend, ora combination thereof. In some embodiments, at least one of the skins isimpregnated with resin either before or after stitching said skins tothe foam core. In some embodiments, at least one of the skins is fibrousprepreg before stitching said skins to the foam core. In someembodiments, at least one protective sheet is fibrous prepreg afterstitching skins to the foam core.

In some embodiments the space surrounding the yarn beams is filled withthe foam of the present invention. In some embodiments the spacesurrounding the yarn beams inside foam core is filled with the foam ofthe present invention, substantially without separate resin binder.

As used hereinthroughout, the term “second foam”, in the context of thesandwich stitched structure, refers to the foam surrounding the yarnbeams.

In some embodiments, the sandwich stitched structure is characterized ashaving second foam which is substantially or approximately the same asthe foam core.

In some embodiments, the sandwich stitched structure is characterized ashaving second foam with a density that is lower than the density of thefoam core. In some embodiments, the sandwich stitched structure ischaracterized as having second foam with a density that is higher thanthe density of the foam core. In some embodiments, the sandwich stitchedstructure is characterized as having second foam with a density that ise.g., at least 5%, at least 10%, at least 15%, at least 20%, at least25%, at least 30%, at least 35%, at least 40%, at least 45%, at least50%, at least 60%, at least 100%, at least 200%, at least 300%, at least400%, at least 800% higher than the density of the foam core.

In some embodiments, the skin comprises ceramic, metal, fibrouscomposites or any combination thereof.

In some embodiments, at least one of the skins comprises a sheet. Insome embodiments, the sheet is a protective sheet. Exemplary sheetsinclude, but are not limited to, moisture protective sheet, an adhesivefilm sheet, a thermoplastic sheet, a metal sheet, a ceramic sheet, and afibrous composite sheet, and any combination thereof. According to someembodiments of the invention, the protective sheets protects againstwater or moisture penetration into microspheres foam. In additionalembodiments, the protective sheets protect against wind, sand erosion,and any factor that may destroy or change properties of microsphere foamwithin the needle pinched holes.

In some embodiments, the combination of the stitched skins and theprotective sheets minimizes the effect of dislocation or destruction ofyarns of fabrics during stitching. In some embodiments, the protectivesheet is useful to strengthen the bonding of yarn beams to skins. Insome embodiments, at least a part of a stitching yarn is located betweenthe skins and/or the protective sheets. In some embodiments, at leastone stitching yarn is supported with at least one protective sheet thatdoes not have holes therein. In some embodiments, the protective sheetscover and protect needle pinched holes. In some embodiments, at leastone pinched holes is filled with microsphere foam.

In some embodiments, a combination of skins and protective sheets areused with a thickness ratio of skins/protective sheets of e.g., about100, about 40, about 20, about 10, about 5, about 1, about 0.5, about0.1, about 0.05, about 0.02, about 0.01, including any valuetherebetween.

In some embodiments, the protective sheet is bonded to at least oneskin. The term “bond” or any grammatical derivative thereof, as usedherein, means adhere into at least one portion in a long lasting manner.

In some embodiments, the protective sheet seals the surface of the skin.In some embodiments, the protective sheet comprises, without limitation,any resin or fibrous material as described or exemplifiedherenthroughout. In some embodiments, the protective sheet comprises apre-preg of woven or non-woven material or a combination thereof.

Without been bound by any particular theory, it is assumed that aprotective sheet being placed on the down face of the sandwich assemblyprevents leaking the microspheres from needle pinched holes.

Further embodiments of this aspect of present embodiments are includedhereinbelow, under the “Method of Fabricating Multilayer Structures”,hereinbelow, and form an integral part of embodiments relating to themultilayer structure as described or exemplified hereinabove.

Reference is now made to FIG. 4 which shows a general schemaillustrating an embodiment of non-filled sandwich panel 100. “Non-filledsandwich panel” 100 refers to a sandwich after stitching, prior tofilling thereof with expandable microspheres.

Non-filled Sandwich panel 100 may have a housing. The housing may bemade of a rigid, durable material, such as, without limitation,aluminum, stainless steel, a hard polymer and/or the like.

Non-filled sandwich panel 100 may have foam core 110. Foam core 110 maycontain one or more needle pinched holes. Non-filled sandwich panel 100may have one or more skins 120. Skins 120 may contain needle pinchedholes. Non-filled sandwich panel 100 may have yarn beams 130. Yarn beams130 may be located in a core. Non-filled sandwich panel 100 may have oneor more protective sheets 140. Protective sheet 140 may close the needlepinched holes. Non-filled sandwich panel 100 may have empty space 150.Empty space 150 may surround one or more elements of sandwich panel 100e.g., yarn beams 130.

Embodiments of the structural elements of sandwich panel 100, e.g.,“sandwich panel”, “needle pinched holes”, “skins”, “protective sheet”,“yarn beams” are described hereinthroughout.

Reference is now made to FIG. 5 which shows a general schemaillustrating an embodiment of a filled sandwich panel 100 a. “Filledsandwich panel” 100 a refers to a sandwich panel after stitching andfollowing filling thereof with expandable microspheres.

Filled Sandwich panel 100 a may have a housing. The housing may be madeof a rigid, durable material, such as, without limitation, aluminum,stainless steel, a hard polymer and/or the like.

Filled sandwich panel 100 a may have foam core 110 a. Foam core 110 amay contain one or more needle pinched holes. Filled sandwich panel 100a may have one or more skins 120 a. Filled sandwich panel 100 a may haveyarn beams 130 a. Yarn beams 130 a may be located in a core (e.g., afoam core). Filled sandwich panel 100 a may have one or more protectivesheets 140 a. Protective sheets 140 a may be located e.g., at a lowerand/or an upper part of filled sandwich panel 100 a so as to close theneedle pinched holes (e.g., the needle pinched holes containingmicrospheres foam based on the disclosed expandable microspheres).Filled sandwich panel 100 a may have space 150 a. Space 150 a maysurround yarn beams 130 a. Space 150 a may contain fused thermoexpandable microspheres. Thermo expandable microspheres may not containa separate resin binder.

Embodiments of the structural elements of sandwich panel 100 a, e.g.,“needle pinched holes”, “skins”, “protective sheet”, “yarn beams” aredescribed hereinthroughout. Embodiments of thermo expandablemicrospheres (also referred to as “thermoplastic expandablemicrospheres”) are also described hereinthroughout.

Reference is now made to FIG. 6 which shows a general schemaillustrating a close view of a fragment of core 600 of stitched sandwichpanels known in the art.

Core 600 may have foam core 610. Core 600 may have cured resin 620.Cured resin 620 may fill an outside space surrounding stitching yarnbeams. This space (e.g., in the form of a hole) may be derived frompenetrating a needle during stitching; Core 600 may have stitching yarns630. Cured resin 620 may be located within stitching yarns (denoted as620 a). This resin bonds stitching yarns and forms stitching yarn beams.

Reference is now made to FIG. 7 which illustrates a close view of afragment of core 700 of the disclosed stitched sandwich panels.

Core 700 may have Foam core 710. Foam core 710 may be based onexpandable microspheres 720. Expandable microspheres may fill an outsidespace surrounding stitching yarn beams. This space (e.g., in the form ofa hole) may be derived from penetrating a needle during stitching. Core700 may have cured resin 730. Core 700 may have stitching yarns 740.Cured resin 730 may be located within stitching yarns 740. Cured resin730 may bond stitching yarns and forms stitching yarn beams.

Embodiments of the structural elements of Core 700 e.g., “stitching yarnbeams”, “expandable microspheres”, “stitching yarns”, and “stitchingyarn beams” are described hereinthroughout.

Method of Fabricating Multilayer Structures

According to an aspect of some embodiments of the present inventionthere is provided a method of preparing multilayer structure(s).

Exemplary multilayer structures include, but are not limited to,sandwich panel, honeycomb structure, truss structure, corrugatedstructure, and 3D woven sandwich structure.

It is noteworthy that typically, the resin might cause overweight ofcomposites, and therefore limiting their use is desirable in manyapplications.

The present inventors have devised and successfully prepared andpracticed stable multilayer structures which do not necessitate a use ofa resin. Nevertheless, in several embodiments of the present invention,as detailed hereinthroughout, the composition may still comprise resin.It is understood to one skilled in the art that by use of the foamand/or polymeric microspheres of the invention, the multilayerstructures disclosed herein are in need of substantially lower amountsof resin as compared to multilayer structures without the foam and/orpolymeric microspheres of the invention.

In some embodiments, a sandwich panel as described herein, in any of theembodiments thereof, including exemplary compositions-of-matter asdescribed herein, is prepared by the steps of: stitching a first and asecond skin to a core of foam using yarn to form an assembly;infiltrating expandable microspheres into the space surrounding theyarn; and heating the assembly. In some embodiments, the steps areperformed sequentially. In some embodiments, the steps are not performedsequentially. In some embodiments, the foam is fabricated at the samestage with the stitching. In some embodiments, the expandablemicrospheres are the pre-expanded microspheres as discussedhereinthroughout.

As used herein, the terms “assembly” or “sandwich assembly” refer to asandwich frame having multiple components or elements, or portionsthereof, being fitted together, prior to setting the final form of thesandwich structure of the present invention.

In some embodiments, foam core is prepared from pre-expandedmicrospheres being arranged uniformly in volume unit. In someembodiments, the pre-expanded microspheres is arranged uniformly bymethods that include, but not limited to, shaking, vibration, rotation,fluidized bed, and any combination thereof.

As used herein, the term “stitching” includes any type of sewing orneedlework. In some embodiments, the stitching is fabricated by manualsewing methods known in the art. In some embodiments, the stitching isperformed using sewing machines. In some embodiments, the sewing machinehas multiply needles. In some embodiments, the sewing machine iscombined with computer programs for higher accuracy of operation andhigh speed of production. In some embodiments, the sewing machine isrobotic.

As known in the art, standard sewing industrial needles have diameterthat ranges from e.g., about 1.5 to about 2.3 mm, while expandablemicrosphere have diameter that ranges from e.g., about 10 to about 200micrometers.

In some embodiments, stitching needles coated with non-sticky coatingare used.

Typically, the yarn stitching may be performed in various patternsincluding, but not limited to, different angles, differentconfigurations and different architectures (e.g., pyramidal or diamondtruss like configuration), in 2D or 3D space.

In some embodiments, the needle pinched holes in the foam core isderived from using a needle during stitching process. The presentinventors have contemplated that expandable microspheres can fill theneedle pinched holes within foam core using infiltration with theassistance of vibration, shaking tools, as described hereinbelow.

In some embodiments, the yarn may be any yarn as described orexemplified hereinabove under the “multilayer structures”.

In exemplary embodiments, the yarn used for stitching skins to the coreof the foam comprises Kevlar 29. In additional exemplary embodiments,the yarn used for stitching skins to the core of the foam comprisesKevlar 149. In additional exemplary embodiments, the yarn used forstitching skins to the core of the foam comprises carbon fiber. Inadditional exemplary embodiments, the yarn used for stitching skins tothe core of the foam comprises carbon T 300 fiber. In some embodiments,the the carbon fiber is characterized by a modulus of e.g., at least 40GPa, at least 80 GPa, at least 120 GPA, at least 140 GPA, at least 200GPa, at least 300 GPa, at least 400 GPa, at least 600 GPa, or at least700 GPa.

In some embodiments, the stitching yarn is impregnated with resin. Insome embodiments, the stitching yarn is impregnated with resin beforestitching the skin. In some embodiments, the stitching yarn isimpregnated with resin after stitching the skin. In some embodiments,the yarn impregnated resin is at least partially cured. In someembodiments, the yarn is twisted. In some embodiments, the yarn isuntwisted. In some embodiments, the yarn is untwisted prior impregnationand twisted after impregnation.

In some embodiments, yarn beams are characterized, without limitation,as having rigid fixed edges. In some embodiments, the yarn beam islocated within the panel core. In some embodiments, a portion of yarnbeam is located on the surface of the skin. In some embodiments, aunidirectional (UD) yarn beam is located within panel core.

In some embodiments, the UD yarn beam is arranged 90° to the skin tothereby maximize the flatwise compression and/or the tensile properties.In some embodiments, the UD yarn beam is arranged in the range of e.g.,about 45°, about 60°, about 70°, about 80°, or about 90°, including anyvalue therebetween, to the skin to thereby optimize the shearproperties. In some embodiments, a portion of UD yarn beam is located onskins and bonded to surface of the skin. In some embodiments, a portionof UD yarn beam is located between skins and protective sheets and isbonded to surface of the skins and the protective sheets. In someembodiments, the yarn beams are arranged in 3D space such that desirableshear, and/or tensile and/or compression properties are exhibited.

In some embodiments, the space around the UD yarn beams within panelcore is filled with the light density (LD) foam as described herein. Insome embodiments, said LD foam is substantially the same as the panelcore foam. In some embodiments, said LD foam is different from the panelcore foam. The present inventors have contemplated that the LD foamimparts an elastic foundation for UD yarn beams located within the core,and/or significantly increases the buckling resistance of the UD yarnbeams and/or the significantly increases the mechanical performance ofsandwich panel at loading.

In some embodiments, slim yarn beams with long length and small crosssection area are used, to thereby optimize the slenderness length and/orradius of the yarn beams.

In some embodiments, the infiltration of the expandable microspheresinto the space surrounding the yarn is performed with assistance of avibration tool. In some embodiments, the infiltration of the expandablemicrospheres into needle pinched holes in foam is performed withassistance of a vibration tool. In some embodiments, a vibration tool isused to vibrate the whole sandwich assembly. In some embodiments, thevibration tool vibrates the skins and the yarns. In some embodiments,the vibration tool vibrates the space surrounding the yarns. In someembodiments, the vibration tool may have frequencies that range from 0.5Hz to 5000 Hz.

In some embodiments, an adhesive film is inserted between the skins andfoam core before stitching processing. In some embodiments, the adhesivefilm comprises a non-sticky resin. In some embodiments, the adhesivefilm comprises sticky resin. In some embodiments, the adhesive filmcomprises any resin that includes, but not limited to, the resins yarnas described or exemplified hereinabove under the “multilayerstructures”. In some embodiments, the adhesive film bonds the skins tofoam core. In some embodiments, the adhesive film seals the spacebetween skins and foam core. In some embodiments, at least one of theskins is covered with at least one protective sheet after stitchingprocessing.

The present inventors have contemplated another technological variantthat uses frames that support skins against deflection during thestitching process. In some embodiments, the skins are fixed on frames.In some embodiments, yarns articles are used to stitch skins. In someembodiments, the profile of the frame is flat or complex. In someembodiments, the frame comprises cells from material similar to the foambetween the skins or cells from honeycomb material for obtaining abetter support to the skins from deflection.

In some embodiments, heating of sandwich assembly is performed. In someembodiments, the heating of sandwich assembly is performed to expand andfuse expandable microspheres. In some embodiments, the heating ofsandwich assembly is performed at e.g., about 50° C., about 60° C.,about 70° C., about 80° C., about 90° C., about 100° C., about 110° C.,about 120° C., about 140° C., about 150° C., about 160° C., about 170°C., about 180° C., about 190° C., about 200° C., about 210° C., about220° C., about 230° C., 240° C., about 250° C., about 260° C., about270° C., about 280° C., about 290° C., 300° C., about 310° C., about320° C., about 330° C., about 340° C., about 350° C., or any valuetherebetween.

In some embodiments, heating the sandwich assembly is performed toprocess the resin of skins. In some embodiments, the heating of sandwichassembly is performed to process the resin of adhesive films. In someembodiments, the heating of sandwich assembly is performed to processthe resin of protective sheets. In some embodiments, the heating ofsandwich assembly is performed to cure resins of sandwich assembly. Insome embodiments, the heating of sandwich assembly is performed to meltthermoplastic components of sandwich assembly. In some embodiments, theheating of sandwich assembly is performed to bond skins to foam core. Insome embodiments, the heating of sandwich assembly is performed to bondskins to foam core with assistance of adhesive films. In someembodiments, the heating of sandwich assembly is performed to bondprotective sheets to skins. In some embodiments, the heating of sandwichassembly is performed in a closed mold. In some embodiments, the heatingof sandwich assembly is performed in an open mold. In some embodiments,the heating of sandwich assembly is performed in a thin-walled tubeshaped mold. In some embodiments, the heating of sandwich assembly isperformed in a rectangular shaped mold.

In some embodiments, the heating of sandwich assembly is performed in amold suitable for sandwich panel shape. In exemplary embodiments, thefusing of the expandable microspheres is performed without separateresin or a binder.

It is noteworthy that resin increases significantly weight. The densityof a resin can be in range of e.g., about 1000 kg/m³ to about 1300kg/m³, while the density of expandable microspheres related to thepresent invention is in the range of, e.g., about 5 kg/m³ to about 50kg/m³. In exemplary embodiments, the density of expandable microspheresrelated to the present invention is in the range of 6 kg/m³ to about 25kg/m³. In additional exemplary embodiments the density of expandablemicrospheres related to the present invention is in the range of about 1kg/m³ to about 25 kg/m³.

It is also noteworthy, without being bound by any particular theory,that the use of mixture of expanded microspheres and resin maycomplicate the manufacturing process since, for example, themicrospheres surfaces might be wettable with the resin, and resin mightcrash or might cause wrinkling of microspheres shells at the heating andcooling processes due to shrinkage of e.g., epoxy or phenolic resin.

It is further noted, without being bound by any particular theory, thatthe expandable microspheres heating creates positive pressure duringtheir heating, supporting the yarn beams at curing process, whileavoiding over-buckling conditions of the beams. The positive pressureeffect at heating is of particular importance when using weak and lowdensity foam that cannot support atmospheric pressure at wide used resinvacuum impregnation processing.

In some embodiments, the microspheres based foam core is reinforced withstochastic fiber filaments. In some embodiments, the stochastic fiberfilaments have a fiber's length longer than fiber critical length withinmicrosphere foam, such as the 3D long fibers as mentioned in U.S. Pat.No. 6,864,297. In some embodiments, the stochastic fiber filaments arein form of webbing with predominantly arranged fiber filaments in onedirection. In some embodiments, the webbing fiber filaments are arrangedpredominantly perpendicular to sandwich skins. In some embodiments, theedges of stochastic fiber filaments are exposed and bonded to sandwichfaces to thereby create bonding contacts between sandwich faces andfiber filaments. In some embodiments, the stochastic fiber filaments areaimed at enhancing the shear and flatwise tensile properties of thesandwich panel. In some embodiments, the edges of stochastic fibers arebonded directly to the skins.

Without being bound by any particular theory it is hypothesized thatfoams with densities below 25 kg/m³, below 10 kg/m³, below 5 kg/m³, andbelow 1 kg/m³ of the present invention have reinforcement effects forstochastic fibers in foam matrix distinguish from foam density of e.g.,100 kg/m³. For example, shells of microspheres, characterized by densitythat ranges from of about 1 kg/m³ to about 25 kg/m³ are thinner and moreelastic that may lead to better enveloping spherical microspheres shellsaround cylindrical fibers and hence exhibiting better physical contactsbetween the microspheres and the fibers. In case of very stiffmicrospheres shells, such as shell comprising glass, the contact areabetween the microspheres and the fibers may be as close as to amathematical point. Therefore, while considering the reinforcementeffect of the fibers one should take into consideration not only themacroscopic properties of the matrix but also the structure and themorphology in the micron level, and the sizes of the microspheres andthe fibers.

The present inventors have contemplated that it is possible to combinethe process of synthesis of foam and with the curing of the resin, or toseparate the processes of foam formation and resin curing.

In some embodiments, expandable or pre-expanded polymeric microspheresare used to fabricate foam within empty spaces of various molds orhoneycomb structures.

In some embodiments, the microspheres are poured into the empty spacesat the mold or honeycomb until e.g., about 10%, about 20%, about 30%,about 40%, about 50%, about 60%, about 80%, about 100%, of the emptyspace is filled. In some embodiments, the empty space is filled withassistance of a vibration tool.

In some embodiments, light density foam surrounding the fiber beams isfabricated. In some embodiments, the light density foam surroundingfiber beams is the same as the core foam.

The present inventors have contemplated utilizing another technologicalapproach which involves a blowing method for fabricating the panel core.

In some embodiments of the blowing method the fabric skins are stitchedone to another and encapsulated forming air hermetic bag. In someembodiments of the blowing method suitable gas is pressured betweenskins forcing skins to move one from another to thereby form air blowingstructure. In some embodiments of the blowing method the expandablemicrospheres are further infiltrated into the air bag and then heated toexpand the microspheres.

In some embodiments the expandable microsphere based foams as usedherein can withstand high temperature. By “high temperature” it is meanttemperature having a value that ranges from e.g., about 80° C. to about700° C.

In some embodiments, the microspheres are mixed with water to fill theneedle punched channels with microspheres, or fill space of a mold orhoneycomb. In some embodiments the microspheres are subsequently driedfrom the water.

In some embodiments water solution with reduced surface tension is used.It is noteworthy that wrinkles of thin and weak microspheres shells oflarge size microspheres may be caused due to the high surface tension ofwater. Wrinkling may lead to reduced performance of microspheres.Hydrocarbon liquid, including, without limitation, n-butanol, n-butan,ethanol, and surfactants can be utilized to reduce surface tension ofwater.

In some embodiments, expandable microspheres are utilized while beingmixed with additives which include, without limitation, flameretardants, smoke depressants, or any combination thereof.

In some embodiments, the quantity of the capillaries is reduced. As usedherein “capillaries” refers to percolated pores between the fusedmicrosphere foam.

In some embodiments, the quantity of the capillaries is reduced byperforming the method of fabrication of sandwich panel, as described inthe Example section.

In some embodiments, microspheres with approximately equal diameters arefabricated by using a size filtration method being applied onmicrospheres with wide size distribution. By “approximately equaldiameters” it is meant equal diameter values with a margin of up toabout ±20%.

In some embodiments, microspheres with approximately equal densities arefabricated using a density filtration method, for example liquid densitycolumn, being applied on microspheres with wide density distribution.

In some embodiments, size and density filtration methods are appliedeither to unexpanded and/or pre-expanded type of expandablemicrospheres.

In some embodiments, the microspheres of the present invention areformed from an expanded or unexpanded type of expandable microspheres,or a mixture thereof or a mixture of expanded and unexpanded type ofexpandable thermoplastic microspheres.

The invented sandwich panel of the present invention can be flat shapedor with a complex curvature shape. The invented sandwich panel of thepresent invention can be fully or partially covered with paint or/andprotective layers.

The invented sandwich panels can be used for applications that include,without limitation, blast, impact resistance, noise damping, aircraftengine nacelle structure, fracture resistance, damage tolerance, andenergy absorption applications. For better blast resistance sandwichpanel can be assembled with additional additives, as noted hereinabove,which include, without limitation, ceramic or metals sheets.

Process of Coating Microspheres with Conductive Additive

According to an aspect of some embodiments of the present inventionthere is provided a process of preparing microspheres or microspherefoam which further comprises any one of the conductive additive asdescribed hereinabove. In some embodiments, the shells of microspheresare coated with conductive additive. Typically, but not exclusively, theconductive additive are dispersed in a medium not harmful to the shellsof the microspheres. In some embodiments, the conductive additive coatsthe shells of the microspheres. In some embodiments, the conductiveadditive forms a network structure on the shell of the microspheres. Insome embodiments, the conductive additive is used to assist the fusingof the shells of the thermoplastic microspheres to thereby form foam. Insome embodiments, the conductive additive is formed on thin elasticshells of the microspheres. In some embodiments, at least one network ofconductive additive coats the shells of the microspheres. In someembodiments, the conductive additive forms a uniform and homogenouslycoating.

By “thin elastic shells” it is meant that the thickness of shells oflarge microspheres are with diameter of about e.g., 50 micron, about 100micron, about 200 micron, about 300 micron, 500 micron and are less thanabout 100 nm in size and are comparable with the sizes of conductiveadditive coating.

In some embodiments, the shells of the microspheres have a nanosizedthickness. In some embodiments, the conductive additive coats at leaste.g., about 10%, about 20%, about 30%, about 40%, about 50%, about 60%,about 70%, about 80%, about 90%, about 90%, of the voids of surfaces ofthe foam.

In some embodiments, the conductive additive is further dispersed in thespaces between the voids of the microspheres of the foam. In someembodiments, the content of the conductive additive within themicrospheres is e.g., at least 1%, at least 5%, at least 10%, at least20%, at least 25%, at least 50%, at least 100%, at least 200%, at least300%, by weight.

Dispersion liquid media usable according to some embodiments of thepresent invention include, but not limited to, an organic liquid, anon-organic liquid, a polar liquid, a non-polar liquid and liquid mediawith any combination thereof.

In some embodiments, the dispersion liquid may include, withoutlimitation, a thixotropic liquid, a gel, colloid, suspension that isviscous under static conditions and flows when shaken or agitated orotherwise stressed. In some embodiments, the liquid medium wetsmicrospheres shells. In some embodiments, the liquid medium diffusesinto microspheres shells. In some embodiments, the liquid medium swellsmicrospheres shells. In some embodiments, the dispersion of theconductive additive is processed in a liquid medium.

In some embodiments, the dispersion of the conductive additive isprocessed at a time—temperature regime in which the liquid medium wetsthe shells of the microspheres but do not dissolve the shells.

As used herein the term “dissolve” refers to the solubility asdetermined by either Hildebrand solubility parameter or the Hansensolubility parameter.

In some embodiments, the dispersion of the conductive additive (e.g.,carbon nanotube) is processed at time—temperature regimes in which aliquid medium swells shells of microspheres but not dissolve the shells.

In some embodiments, the liquid medium has surface tension that rangese.g., from about 10 dyn/cm to about 50 dyn/cm, from about 5 dyn/cm toabout 60 dyn/cm, from about 1 dyn/cm to about 80 dyn/cm.

Exemplary liquid media include, but not limited to, ethyl ethanol,toluene, tetrahydrofuran, hexane, water, aqueous medium, water with asurfactant, and any combination thereof. In some embodiments, the liquidmedia further include trifluoro acetic acid.

Exemplary surfactants include, but not limited to, cationic, anionicionic and non-ionic surfactant, short and long chain polymericsurfactants. Additional exemplary surfactants include, but not limitedto, sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, sodiumBenzoate, Gum Arabic, Brij S-100, didodecyldimethylammonium bromide,Pluronic F-127, polyvinylpyrrolidone; vinylpyrrolidone, n-methyl-2pyrrolidone, N,N dimethyl propylene, sodium cholate, sodiumdeoxycholate, Triton X-100, Triton X-405, Tween 60, pluronicssurfactants, P123, F68, F108, F88, F127, hexadecyltrimethylammoniumbromide, cetyl trimethylammonium bromide, Na-cholate, surfynol CT324,Aerosol OS, Dowfax 2A1.

In some embodiments, dispersants soluble or partially soluble are used.In some embodiments, the dispersants are not harmful to the shells ofmicrospheres.

As used hereinthroughout, the term “not harmful” refers to the relevantstandards determined by the vendor of microspheres, and/or to thestandards known in the art.

In some embodiments, the conductive additive is conductive polymers. Insome embodiments, the conductive additive is intrinsically conductivepolymers soluble or partially soluble in liquids that are not harmful tothe shell of microspheres.

Exemplary conductive polymers further include, but not limited to,polyacetylene, polypyrrole, polyaniline, poly(fluorene)s,polyphenylenes, polypyrenes, polyazulenes, polynaphthalenes,poly(pyrrole)s, polycarbazoles, polyindoles, polyazepines, polyanilines,poly(acetylene)s, poly(p-phenylene vinylene), poly(3-hexylthiophene),poly(thiophene)s, poly(3,4-ethylenedioxythiophene), poly(p-phenylenesulfide) or their copolymers, or their metal-doped form, and anycombination thereof.

In some embodiments, the conductive polymers are characterized as havingpolymeric network. In some embodiments, the polymeric network compriseslong chain polymer. In some embodiments, the polymeric network compriseslong chain polymer and short chain surfactants. In some embodiments, thepolymeric network is in the form of a gel.

In some embodiments, surfactant is used to reduce the surface tension ofthe water. In some embodiments, a water azeotrope is used to reducesurface tension of water. Exemplary azeotrope may include, but notlimited to, water—n-butanol mixture. In some embodiments, the conductiveadditive is dispersed in a low viscous liquid medium.

In some embodiments, the conductive additive is dispersed in a highviscous liquid medium. In some embodiments, the conductive additive isdispersed in low viscous liquid containing microspheres.

In some embodiments, the liquid medium (e.g., organic liquid) isselected so as to disperse the additives (e.g., carbon nano tubes). Insome embodiments, the method does not include use of surfactants, so asto avoid effects of surfactants on electrical percolation of the carbonnano tube additives.

In some embodiments, the organic liquid(s) is evaporated fully.

Without being bound by any particular theory, the activity of the liquidwas selected to wet or slightly swell the shells of the microspheres,without dissolving the shells.

In some embodiments, the liquid in which the carbon nano tubes aredispersed have a similar Hildebrand solubility parameter to thepolymeric shells of microspheres.

It is to note that, in some embodiments, the ratio of liquid tomicrospheres may be determined to affect wetted layers of liquidcontaining CNT around microspheres shells. The viscosity of thesuspension may be controlled to prevented re-agglomeration of CNTs.

In some embodiments, the dispersion of the conductive additive in theliquid is assisted by e.g., ultrasonic irradiation, planetary mixer, ablender, a ball mill, or a combination thereof. Typically, ultrasonicirradiation is less suitable to apply on high viscosity medium, but itis suitable to use planetary mixing.

Fabrication of Powder of Microspheres Coated with the ConductiveAdditive

According to an aspect of some embodiments of the present invention,powder of the microspheres is coated with any of conductive additive asdescribed hereinabove (e.g., nanoparticles or carbon nanotubes).

In some embodiments, unexpanded microspheres are coated with aconductive additive. In some embodiments, pre-expended microspheres arecoated with nanoparticles. In some embodiments, a mixture of unexpandedand pre-expended microspheres is coated with nanoparticles. In someembodiments, the coating performed on unexpanded microspheres isfollowed with heat expansion. In some embodiments, the coating performedon pre-expanded microspheres is followed with heat expansion.

In some embodiments, the carbon nanotubes are dispersed in a liquid. Insome embodiments, the dispersed carbon nanotubes and microspheres aremixed. In some embodiments, powder of microspheres is dispersed in lowviscous liquid medium containing carbon nanotubes.

As exemplified in the Example section that follows, liquid medium withhigh surface tension may lead to shrinkage and wrinkling ofmicrospheres. The observed shrinkage and wrinkling effects may diminishcompression properties of microspheres. The shrinkage and wrinkling maybe repaired by heating.

Typically, the volume liquid/microspheres ratio influences the viscosityof the suspension containing microspheres; suspension containingmicrospheres and liquid connected with microspheres is characterized byhigh viscosity; suspension with not packaged microspheres and freeliquid (i.e. not connected to the microspheres) is characterized by lowviscosity.

In some embodiments, the mixing of the microspheres and the liquidmedium containing carbon nanotubes is assisted by methods that include,but not limited to, stirring, shaking, vibration, rotation, mill balls,blending, and high speed mixing. In some embodiments, the microspheresare immersed in the liquid containing dispersed carbon nanotubes withoutfurther mixing this suspension. In some embodiments, the microspheresare coated following an immersion.

In some embodiments, the suspension containing carbon nanotubes andmicrospheres is dried. In some embodiments, the suspension is dried bymethods that include, but not limited to, heating, vacuum, filtration,centrifugation, removing the liquid medium by extraction. In someembodiments, the suspension is dried with the assistance of shaking.Typically, shaking may be performed at different frequencies,amplitudes, and configurations of basis motion.

In some embodiments, the carbon nanotubes may be coated fully orpartially by electro conductive polymer molecules.

In some embodiments, at drying, the temperature of the liquidevaporation is below glass transition of microspheres. In someembodiments, the temperature of the evaporation is below the melttransition of the microspheres.

In some embodiments, after coating the shells of microspheres withcarbon nanotubes, the non-coating carbon nanotubes are washed out by,e.g., a surfactant, a dispersant, or a combination thereof.

In some embodiments, the surfactant, dispersant, or any combinationthereof, is further washed out with the assistance of another liquid,e.g., polar or non-polar liquid. In some embodiments, the surfactant,dispersant, or any combination thereof, is further washed out wash outwith the assistance of e.g., sulfuric acid, nitric acid, ultrasonicirradiation, UV, an enzyme, or any combination thereof.

In some embodiments, the sulfuric acid, or the nitric acid, is in aconcentration that ranges from about 1 to about 20, moles per liter(mol/L).

In some embodiments, the carbon nanotubes form a structure of e.g., thincoating, mono layers, on the shells of microspheres. In someembodiments, a network of carbon nanotubes is formed on the shells ofmicrospheres. In some embodiments, the network of carbon nanotubes is informs that included, but not limited to, spots of carbon nanotubes ontoshells, dispersed individual carbon nanotubes, dispersed individualstretched carbon nanotubes, dispersed carbon nanotubes arranged withbundles of parallel carbon nanotubes.

In some embodiments, the electrical conductivity of the carbon nanotubesis improved by a formation of bundles of parallel oriented carbonnanotubes that are characterized by better contact between individualcarbon nanotubes.

In some embodiments, the carbon nanotubes form a structure of monolayers, derived from e.g., Van-der-Waals interactions between thenanotubes and the polar molecules of the microspheres shells, to therebyform dispersed networks of the carbon nanotubes on the shells and toavoid agglomerations of particles.

In some embodiments, the networks of the dispersed conductive additiveon the shells of microspheres lead to enhanced electrical conductivityof the microspheres.

In some embodiments, the networks of the dispersed conductive additiveon the shells of microspheres lead to enhanced mechanical performance ofthe microspheres.

In some embodiments, carbon nanotubes are deposited on shells ofmicrospheres in-situ, during synthesis of the carbon nanotubes. In someembodiments, the carbon nanotubes are in-situ deposited in a structureof networks of bundles of carbon nanotubes on the surface of themicrospheres.

In some embodiments, the coating of microspheres is performed in specialdesigned reactor. In some embodiments, the coating of the microspheresis performed directly in mold.

Typically, but not exclusively, when the coating of the microspheres isperformed directly in mold, the liquid is dried in the mold to therebyfabricate the foam after the drying of the liquid without subsequentremoval of the coated microspheres from mold.

As demonstrated herein, a powder of microspheres coated with carbonnanotubes was fabricated. FIG. 2 shows a SEM image SEM imageillustrating a fragment of a pre-expanded microsphere coated withmultiwall carbon nanotubes.

In some embodiments, the quantity of the carbon nanotubes on the shellsof microspheres varies in the powder.

In some embodiments, the conductive passes within are incorporated onthe disclosed microspheres. In some embodiments, the foam comprises oneor more conductive passes. In some embodiments, the conductive passesare incorporated within capillaries of the disclosed foam.

As described herein, in some embodiments, the foam is fabricated frompristine pre-expanded microspheres. As further described herein, in someembodiments, the foam is fabricated from pristine uniform density oruniform size pre-expanded microspheres. In some embodiments, the foamcomprises fused spots between pristine microspheres.

In some embodiments, the dispersion of conductive nanoparticles isinfiltrated into capillaries of foam. In some embodiments, thedispersion of conductive nanoparticles is deposited on the shells ofmicrospheres.

In some embodiments, the dispersion of nanoparticles substantially doesnot penetrated fused spots of microspheres.

In some embodiments, conductive nanoparticles form a coating layerwithin the capillaries of the foam. In some embodiments, conductivenanoparticles form conductive passes within the capillaries of foam.

In some embodiments, 2D or 3D patterns of conductive passes ofconductive nanoparticles within foam may be fabricated using a 3Dprinter. One cartridge may utilize pre-expanded microspheres and anotherink/dispersion of nanoparticles. Ink may be dried before heat fusing ofmicrospheres. In some embodiments, the impedance of conductive passes isfabricated according to DC or AC requirements

Honeycomb Structure

In some embodiments, the multilayer structure is in a form of ahoneycomb. In some embodiments, the sandwich panel comprises ahoneycomb.

As defined herein and further known in the art, honeycomb structures areof configurations that include, but not limited to, hexagonal, square,flex-core, double flex-core, spirally wrapped, cross-core, and tubularcore.

In some embodiments, the honeycomb is fabricated from materials thatinclude, but not limited to, metals, steel, aluminum, titanium, aramidfiber paper, carbon fiber paper and thermoplastics.

In some embodiments, the honeycomb comprises the polymeric microspheresof the present invention. In some embodiments, the honeycomb comprisesthe foam of the present invention.

In some embodiments, the polymeric microspheres at least partially fillat least one side of the honeycomb. In some embodiments, the polymericmicrospheres at least partially fill both sides of the honeycomb. Insome embodiments, the polymeric microspheres fill part, or all cells ofthe honeycomb. In some embodiments, the foam at least partially fillseither one side of the honeycomb. In some embodiments, the foam at leastpartially fills both sides of the honeycomb. In some embodiments, thefoam fills part, or all cells of the honeycomb.

In some embodiments of the present invention, cells of honeycombs arefilled with light density microspheres. In some embodiments, the foam isformed within the honeycomb cells.

In some embodiments of the present invention, it is possible to fill thecells of honeycombs with pre-expanded or mixture of pre-expanded andexpandable microspheres.

In some embodiments of the present invention, foam may be fabricated inthe honeycomb placed in closed mold. In some embodiments, foamfabrication within cells of honeycomb and bonding of skins to honeycombscan be performed at once at the same temperature-time regimes, therebysimplifying the processing. In some embodiments, it is possible to fillthe cells of honeycomb, partially by height, with non-expandedmicrospheres. In some embodiments, pre-expanded microspheres partiallyfilling the cells of the honeycombs expand following heating and formfoam. In some embodiments, the foam may have gradient of density indirection of its expansion.

In some embodiments, honeycomb filled with light density foam ischaracterized as having improved properties that include, but notlimited to, noise damping, heat transfer and mechanical damage toleranceperformance.

In some embodiments, the cells of the honeycomb are filled with coatedconductive microspheres and conductive foam may be fabricated within thecells of the honeycombs. In some embodiments, foam may be formedpartially within the volume of the honeycomb. In some embodiments,conductive foam may form uniform, layers, 2D or 3D conductive patternswithin the volume of the honeycomb.

Process of Preparing Pre-expanded Microspheres

According to an aspect of some embodiments of the present inventionthere is provided a process of preparing low density pre-expandedexpandable polymeric microsphere powder, obtained by processingexpandable polymeric microspheres.

In some embodiments, the process comprises a step of heating expandablemicrospheres to temperature of e.g., about 80° C., about 100° C., about120° C., about 140° C., about 160° C., about 180° C., about 200° C.,about 220° C., about 240° C., about 250° C., about 280° C., about 300°C., about 320° C., about 340° C., about 400° C., or any valuetherebetween.

In some embodiments, the heating is performed for time duration of e.g.,about 3 seconds, 30 seconds, about 1 minute, about 2 minutes, about 5minutes, about 20 minutes, about 30 minutes, about 40 minutes, about 60minutes, or any value therebetween. In some embodiments, the heating isperformed while stirring.

In some embodiments, the process of preparing low density pre-expandedexpandable polymeric microsphere powder comprises a subsequent step ofcooling the expandable microspheres to temperature of about e.g., 80°C., about e.g., 70° C., about 60° C., about 50° C., about 40° C., about30° C., about 20° C., about 10° C., or any value therebetween. In someembodiments, the cooling is performed while stirring.

In some embodiments, the cooling is performed in a cooling rate of e.g.,about 5° C./minute, about 10° C./minute, about 15° C./minute, about 20°C./minute, about 25° C./minute, about 30° C./minute, about 35°C./minute, about 50° C./minute, or any value therebetween.

In some embodiments, the process of preparing low density pre-expandedexpandable polymeric microsphere powder comprises a subsequent step ofselecting the expandable microspheres according to size or/and densitydistribution.

In some embodiments, the step of selecting the expandable microspheresfor distribution of sizes or/and densities is performed by methods thatinclude, without limitation, shaking, vibration, rotation, usingfluidized bed, filtration, centrifugation liquid density column, and anycombination thereof.

In exemplary embodiments, the liquid density column comprises a mixtureof water (e.g., 12.5 gram) and ethanol (e.g., 8.7 gram).

Articles of Manufacture

According to an aspect of some embodiments of the present inventionthere is provides an article-of-manufacturing which comprises thecomposition of matter as described herein. Any article that may benefitfrom the polymeric microspheres and/or foam of the compositions ofmatter described herein is contemplated.

Exemplary articles of manufacturing include, but are not limited to, asandwich structure, an electromagnetic interference (EMI) shielding, ananti-radar-shielding, an antenna, an antenna, and a circuit board.

Additional exemplary articles of manufacturing include, but are notlimited to, transportation structures, including, without limitations,noise damping, aircraft engine nacelle structure, sport cars, vibrationand/or crack, impact resistance structures, airframe structure, andaerospace structure.

Additional exemplary articles of manufacturing include, but are notlimited to, heat-isolating structures, damage tolerance structures,fracture resistance structures, blast resistance shielding, bullet andfragment resistance shielding.

Additional exemplary articles of manufacturing include, but are notlimited to, compositions with thermoplastics, compositions withthermosets, underbody coatings, wall coatings, papers compositions,non-woven, filling of tennis balls, crack fillers, spackling compounds,sealant, concrete, paints, shoe soles, printing inks, genuine leather,cosmetics and the like.

Examples below demonstrate some invented technology approaches, someproperties of invented sandwich panels with comparison of literature,open data as well as highlight the ideas and advantages of invention.The invention is not limited to the examples presented.

General

It is to be understood that the invention is not necessarily limited inits application to the details set forth in the following description orexemplified by the Examples. The invention is capable of otherembodiments or of being practiced or carried out in various ways.

It is expected that during the life of a patent maturing from thisapplication many relevant microsphere-based low-density structure willbe developed and the scope of the term low-density structure is intendedto include all such new technologies a priori.

As used herein the term “about” refers to ±10%

The terms “comprises”, “comprising”, “includes”, “including”, “having”and their conjugates mean “including but not limited to”.

The term “consisting of means “including and limited to”.

The term “consisting essentially of” means that the composition, methodor structure may include additional ingredients, steps and/or parts, butonly if the additional ingredients, steps and/or parts do not materiallyalter the basic and novel characteristics of the claimed composition,method or structure.

The word “exemplary” is used herein to mean “serving as an example,instance or illustration”. Any embodiment described as “exemplary” isnot necessarily to be construed as preferred or advantageous over otherembodiments and/or to exclude the incorporation of features from otherembodiments.

The word “optionally” is used herein to mean “is provided in someembodiments and not provided in other embodiments”. Any particularembodiment of the invention may include a plurality of “optional”features unless such features conflict.

As used herein, the singular form “a”, “an” and “the” include pluralreferences unless the context clearly dictates otherwise. For example,the term “a compound” or “at least one compound” may include a pluralityof compounds, including mixtures thereof.

Throughout this application, various embodiments of this invention maybe presented in a range format. It should be understood that thedescription in range format is merely for convenience and brevity andshould not be construed as an inflexible limitation on the scope of theinvention. Accordingly, the description of a range should be consideredto have specifically disclosed all the possible subranges as well asindividual numerical values within that range. For example, descriptionof a range such as from 1 to 6 should be considered to have specificallydisclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numberswithin that range, for example, 1, 2, 3, 4, 5, and 6. This appliesregardless of the breadth of the range.

Whenever a numerical range is indicated herein, it is meant to includeany cited numeral (fractional or integral) within the indicated range.The phrases “ranging/ranges between” a first indicate number and asecond indicate number and “ranging/ranges from” a first indicate number“to” a second indicate number are used herein interchangeably and aremeant to include the first and second indicated numbers and all thefractional and integral numerals therebetween.

As used herein the term “method” refers to manners, means, techniquesand procedures for accomplishing a given task including, but not limitedto, those manners, means, techniques and procedures either known to, orreadily developed from known manners, means, techniques and proceduresby practitioners of the chemical, pharmacological, biological,biochemical and medical arts.

As used herein, the term “treating” includes abrogating, substantiallyinhibiting, slowing or reversing the progression of a condition,substantially ameliorating clinical or aesthetical symptoms of acondition or substantially preventing the appearance of clinical oraesthetical symptoms of a condition.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination or as suitable in any other describedembodiment of the invention. Certain features described in the contextof various embodiments are not to be considered essential features ofthose embodiments, unless the embodiment is inoperative without thoseelements.

Various embodiments and aspects of the present invention as delineatedhereinabove and as claimed in the claims section below find experimentalsupport in the following examples.

EXAMPLES

Reference is now made to the following examples, which together with theabove descriptions illustrate some embodiments of the invention in anon-limiting fashion.

Materials

Kevlar 29 yarns (170 tex; elastic modulus: 70 GPa); Ethanol; Phenolicresin; Epoxy glue Propoxy 20; 551 DE expandable polymeric hollowmicrospheres, xpancel Inc.; 093 DU120 expandable polymeric hollowmicrospheres, Expancel Inc.; Carbon T300 yarns (800 tex; elasticmodulus: 230 GPa); Nanocyl 7000 pristine multi wall carbon nanotubes,Nanocyl Co.).

Example 1 Preparation of Pre-expanded Microspheres

Glass bottles with flat bottom were used as reactors to fabricate apowder of pre-expended microspheres (with a bulk density in the range of5.6-15 kg/m³).

In exemplary procedures, 0.5 grams of unexpanded microspheres (e.g., 093DU120) were placed in a flat bottom bottle covered with aluminum foildeposited on a flat heating plate. The bottle was heated for a fewminutes from temperature of about 140 to 180° C., while stirring with amagnetic stirrer. The powder was thereafter cooled on a cold plate whilestirring. The bulk density of the powder of the microspheres was 5.6kg/m³.

In exemplary procedures, 0.5. gram of unexpanded microspheres 093 DU120was placed in a 250 ml flat bottom bottle (65 mm diameter) covered withAluminum foil. The bottle was deposited on a hot plate while stirringwith a magnetic stirrer, and was heated from 120 to 180° C. for 3minutes, and was thereafter cooled (at a cooling rate of about 10° C.per min) while stirring. Minor and weak agglomerates of microsphereswere observed. The bulk density of the fabricated powder of thepre-expanded microspheres was about 5.9 kg/m³.

In exemplary procedures, 1.5 gram of unexpanded microspheres 093 DU120was placed in a 250 ml flat bottom bottle (65 mm diameter) covered withAluminum foil. The bottle was deposited on a hot plate while stirringwith a magnetic stirrer, and was heated from 135 to 160° C. for 3minutes, and was thereafter cooled (at cooling rate of about 10° C. permin) while stirring. The bulk density of the fabricated powder ofmicrospheres was 15 kg/m³.

It is noteworthy that, as shown in SEM image in FIG. 1, some of the lowdensity pre-expanded microspheres have sizes larger than 200-250 micronsillustrating the very wide distribution of sizes for some accidentalspot sample of pre-expanded powder of microspheres with bulk densityabout 10 kg/m³.

As further demonstrated in FIG. 1 the powder of pre-expandedmicrospheres at bulk density of 10 kg/m³ has some portion ofmicrospheres with diameter about 200 microns. Microspheres having smalldiameters of a few dozen microns are further visible. It is assumed thatthese small microspheres were not expanded properly due to lack ofinternal quantity of gas encapsulated at preliminary synthesis byproducer of microspheres. Small microspheres may contribute andsignificantly increase the density of microspheres foam.

In exemplary embodiments, the small microspheres were extracted from thepowder, so as to prepare a powder comprising narrow fractions of largesize microspheres.

In further exemplary embodiments, powder of pristine pre-expandedmicrospheres was further fabricated. Dry pre-expanded microspheres werefiltrated with assistance of vibration and shaking through filter tubescenter disc Pyrex 40 mm dia. 100-120 microns. Microspheres withdiameters more than 100-120 microns were separated on filter surface andmicrospheres having less 100-120 microns diameters were infiltratedthrough filter.

Pristine foam based on microspheres with diameter more than 100-120microns was fabricated according to Example 4 below. The thickness ofthe microspheres shells was estimated to be about 100 nm and bulkdensity of microspheres powder was estimated to be about 4 kg/m³.

The pre-expanded microspheres characterization was performed onpre-expanded microspheres mixed with high surface tension liquid(water), moderate tension liquid (ethyl ethanol) and low surface tensionliquid (n-hexane).

In exemplary procedures, ethyl ethanol, n-hexane, water were poured intopowder of pre-expanded microspheres fabricated from commercial grade ofunexpanded microspheres Expancel 093 DU 120. Unexpanded microsphereswere heated and stirred into lab reactor within temperature range 120°C. to 160° C. for 3 minutes. Cooling was performed at 10° C. rate withcontinuous stirring. The bulk density of fabricated pre-expanded powderwas about 6.7 kg/m³. 1.3 gram of n-hexan, or ethyl ethanol or water waspoured into 0.04 gram of pre-expanded powder.

The amount of 1.3 gram was selected since this is the maximum quantityof liquid that had filled the spaces between the microspheres withoutfree liquid being found outside the powder.

The volume of each liquid was estimated according to the correspondingdensity: the density of water—1 g/cc, the density of n-hexane—0.65 g/cc,the density of ethanol—0.79 g/cc.

The following calculations are performed to evaluate the free space ofthe microspheres:

Bulk density of microspheres/density of water=0.0067

Bulk density of microspheres/density of n-hexane=0.01

Bulk density of microspheres/density of ethanol=0.0085

Volume water/bulk volume microspheres=(weight water/densitywater)/(weight microspheres/density microspheres)=0.21

Volume ethanol/bulk volume microspheres=0.28

Volume n-hexane/bulk volume microspheres=0.33

It is therefore assumed that free space between packed microspheresballs is in range about 30% to 40% (v/v) since the volume of n-hexanebetween microspheres is close to free space of packed sphericalmicrospheres. The term “packed” and the values were taken from the knownart for dense packed spheres.

It is noted that the microspheres are considered as having sphericalshape. It is further noted that hexan is non polar liquid with lowsurface tension and poor interaction with microspheres shells, whereaswater is polar liquid, has good interaction with microspheres shells andhas high surface tension, and hence, the volume of the water was lowerthan the free space between packaged balls. It is assumed, without beingbound by any particular theory, that the high surface tension of watersqueezes the very thin elastic shells of microspheres.

Ethanol showed intermediate results between hexane and water.

It is further assumed, without being bound by any particular theory,that the bulk density of the microsphere powder filled out volume of themold predetermines the density of the foam, while taking into theaccount the loss of some quantity of gas diffused out of microspheresshells during the heating process.

By mixture of water/methanol in a ratio of about 1.4 by weight (e.g.,12.5 gr of water and 8.7 gram of ethanol; ethanol was added to reducethe density of the liquid) the separation of the pre-expandedmicrospheres by density was observed: a portion of the microspheresfloated whereas the other portion of the microspheres sunk in themixture. It is assumed that water/ethanol ratio manages the lightness offloating portion of microspheres as floating microspheres are lighterand have larger diameters.

Foam samples were prepared on portions of floating and sunk microspheresafter drying microspheres. Foam based on floated microspheres wascharacterized by a lighter density.

Drying Process of Suspensions of Fabricated Microspheres:

The drying of water from the corresponding microsphere suspensionresulted in a slight shrunk of the microspheres and the microspheresremained slightly stuck to each other. The drying of hexane from thecorresponding microsphere suspension did not result in the shrinkingeffect and the microspheres remained free and unstuck. The drying ofethanol from the corresponding microsphere suspension resulted inintermediate results between hexane and water.

Without being bound by any particular theory it is hypothesized thatliquids with high surface tension squeezes the elastic thin shells ofthe microspheres and stuck them to each other after the drying process.In low surface tension liquids, on the other hand, allow themicrospheres to remain unstuck.

It was therefore observed that suspending of the microspheres in water(i.e. liquid with high surface tension) had resulted in the shrinkageand wrinkling of microspheres. Taking into consideration thatmicrospheres are practically ideal thin shell balls and theircompression properties are determined by the buckling effects of thethin shells it can be concluded that observed shrinkage and wrinklingeffects may diminish compression properties of microspheres. Shrinkageand wrinkling may be repaired at heat expansion of microspheres.

Example 2 Coating Pre-expanded Microspheres with Carbon Nanotubes

In exemplary procedures, the pre-expanded microspheres as preparedhereinabove were coated with multi wall carbon nanotubes (Nanocyl 7000pristine multi wall carbon nanotubes produced by Nanocyl Co.).

In exemplary procedures, the coating was performed in n-hexane. Inexemplary procedures, 1.6 g hexane was poured into 0.05 g pre-expandedmicrospheres. 0.025 g of the carbon nanotubes was then added. Thesuspension obtained was then mixed using high speed planetary mixer for10 minutes at 800 rpm to 2500 rpm, while varying the speed every 1minute.

FIG. 2 shows SEM image illustrating a fragment of a pre-expandedmicrosphere coated with multi wall carbon nanotubes.

Example 3 Fabricating Microsphere Foam

Pre-expanded microspheres as prepared hereinabove were poured into moldfilling all its volume.

In exemplary procedures, the mold was next closed and was heated for 7minutes at temperature in the ranges of 140° C. to 180° C., and wasthereafter cooled at about 10° C.

Example 4 Preparation of Shaped Samples of Foam

Fabricating Low-density Microsphere Foam:

Cylindrical foam sample was fabricated using aluminum thin-walled tubemolds with wall thickness of about 1.5 mm, internal diameter of about 22mm and a length of about 50 mm.

One side of an aluminum tube mold was covered by 1 mm thickness aluminumplate. Pre-expanded microsphere powder with a density of 5.9 kg/m³ wasthen poured into the tube with assistance of vibration to fill the tubespace. Next, another side of the mold was covered with a thin aluminumplate as well. The tube mold was inserted into pre-heated hot oven within-built air circulation “Memmert”. The temperature was set at 191° C.,being controlled with accuracy of ±3° C., for 16 minutes. The mold wasthereafter cooled in air at a cooling rate of about 10° C. per min.

The foam sample obtained had a density of 5.2 kg/m³.

A Rectangular Shape Foam Samples:

A rectangular shape foam samples with dimensions 10 mm×10 mm×20 mm werefabricated using an aluminum mold with wall thickness of about 10 mm.

In exemplary procedures, one side of the mold was covered with about 1mm thickness aluminum plate. Pre-fabricated pre-expanded microspherespowder was poured into the tube with assistance of vibration to therebyfill the mold space. Another side of the mold was thereafter coveredwith thin aluminum plate as well. The mold was then heated using a hotpress technique; the mold was placed between pre-heated hot plates and athermocouple was inserted into the mold wall while registering thetemperature to ranges of 146-179° C. for 10 minutes, at a heating rateof about 3° C. per min. The mold was thereafter cooled in air at acooling rate of 10° C. per min. Foam samples with density range of 5-20kg/m³ were fabricated.

It is noteworthy, without being bound by any particular theory, that thedensity of the foam is predetermined by the weight of the powder withinmold minus 8-12% to reduce the weight of gas diffusion frommicrospheres.

It is noteworthy, however, that the diffusion of the gas frommicrospheres is a function of many parameters and the exact quantity ofreduce of weight is determined by experimental pre-trials. Typically, 8to 10 minutes of heating results in reduce of about 8%-10% of weight and15 to 20 minutes of heating results in reduce of about 12% to 10% ofweight.

Cylindrical Foam Samples:

Cylindrical foam samples were fabricated using aluminum thin-walled tubemold with wall thickness about 1.5 mm, internal diameter was about 22 mmand length about 50 mm.

In exemplary procedures, one side of the tube was covered with aluminumplate (1 mm thickness) and pre-fabricated microsphere powder was pouredinto tube with assistance of vibration mean filling in the mold space.

The other side of the mold was then covered with thin aluminum plate aswell and the mold was deposited in pre-heated hot oven with in-built aircirculation “Memmert”. The mold was then heated to 191° C. (±3° C.) for10-20 minutes and was thereafter cooled down on air at a cool rate ofabout 10° C. per min.

The densities of the foam samples were in the ranges of: 5-20 kg/m³.

The density of the foam was predetermined by the weight of the powderwithin mold minus 8-12%, so as to reduce the weight of gas diffusionfrom microspheres.

Morphological Characterization of the Fabricated Microsphere Foam:

A foam sample as prepared hereinabove was broken at room temperature anda fragment thereof was examined in high resolution scanning electronmicroscopy HR SEM Ultra Plus Gemini (Zeiss Co., Germany). Directmeasurements were performed and no carbon or gold coating was applied.

FIG. 3 demonstrates HRSEM image of the foam sample, showing visiblemicrosphere having a diameter of about 260 microns, and a brokenmicrosphere with a diameter of about 170 microns. The thickness of theshell of broken microsphere is estimated as being below 100 nanometers.

Example 5 Sandwich Panel Preparation

Stitching Kevlar 29 Yarns to Bakelite Plates:

In exemplary procedures, the Kevlar 29 yarns were impregnated with 25wt. % ethanol phenolic resin solution and dried for 24 hours at roomtemperature to evaporate the ethanol. After the treatment the yarnscontained about 25 wt. % uncured phenolic resin.

In exemplary procedures, the flexible yarns containing the uncuredphenolic resin were stitched manually through holes of two Bakeliteplates fixed on the metal frame.

In exemplary procedures, for one set of the samples the height of theframe was 20 mm and for another set of the samples the height of theframe was 10 mm. The distance between the holes on the plates was 2.3mm. The stitching yarns were arranged perpendicular to plates.

In exemplary procedures, the phenolic resin was next cured for 1 hour at150° C., and the plates with the stiff fiber beams were separated fromthe frame. Thin (1 mm) layer of fast-curing epoxy glue Propoxy 20 wasplaced onto surfaces of the plates to fix the fiber beams to thesurfaces of plates. The density of the core samples was evaluated as0.045 g/cc.

Infiltrating microspheres into the space surrounding Kevlar 29 yarnbeams and preparation the foam surrounding the yarn beams:

In exemplary procedures, the sample obtained was placed in a closed moldand expandable polymeric hollow microspheres Expancel 551 DE were pouredinto the closed mold and filled the space among the fiber beams. Onceinfiltrated, the assembly was heated in the closed mold to temperatureof about 130° C. for 15 minutes. The microspheres were expanded andfused together, forming foam. The density of the microspheres based foamwas 0.025 g/cc. The density of Kevlar fiber beams samples was evaluatedas 0.045 g/cc. The overall density of the sample was 0.07 g/cc.

Preparation of the Foam:

In additional exemplary procedures, the pre-expanded microspheresExpancel 551 DE were poured into the closed mold and filled the wholevolume of closed mold. The closed aluminum mold was heated in Lab-LineDuo-Vac Oven at a temperature of about 130° C. for 15 minutes. Themicrospheres were expanded and fused together, forming foam. The densityof microspheres based foam sample was 0.025 g/cc. The mold was ofcylindrical shape with internal diameter of 31 mm, outer diameter of 45mm and length of 100 mm. The mold was closed with aluminum covers with22 mm thick.

Stitching Carbon T 300 Yarns to Aluminum Plates:

In exemplary procedures, Carbon T 300 yarns were impregnated with 25 wt% ethanol phenolic resin solution and were thereafter dried for 24 hoursat room temperature to evaporate the ethanol. After the treatment, theyarns contained about 25 wt. % phenolic resin. The Carbon yarnscontaining uncured phenolic resin were stitched manually through holesof two aluminum plates fixed on the metal frame. For one set of thesamples the height of the frame was 20 mm and for another set of thesamples the height of the frame was 10 mm.

In exemplary procedures, the stitching yarns were arranged perpendicularto plates. The distance between holes on plates was 3.6 mm. The phenolicresin was next cured for 1 hour at 150° C. and the plates with stifffiber beams were separated from the frame. Thin (1 mm) layer offast-curing epoxy glue Propoxy 20 was next placed onto surfaces ofplates to fix fiber beams to the surfaces of plates.

The density of the core sample was evaluated as 0.075 g/cc.

Infiltrating Microspheres into the Space Surrounding T 300 Yarn Beamsand Preparation the Foam Surrounding the Yarn Beams:

In exemplary procedures, the sample obtained was placed in a closed moldand expandable polymeric hollow microspheres, Expancel 551 DE, werepoured into the closed mold and filled the space among fiber beams. Onceinfiltrated, the assembly was heated in the closed mold to a temperatureof about 130° C. for 15 minutes to expand the microspheres and to fusethem together, forming foam.

The density of microspheres based foam supporting yarn beams was 0.025g/cc. The density of carbon fiber beams samples was evaluated as 0.075g/cc. The overall density of sandwich core sample was 0.1 g/cc.

Stitching Carbon T 300 Yarns through Foam Core to Glass Skins:

In exemplary procedures, Carbon T 300 yarns were impregnated with 25 wt.% ethanol phenolic resin solution and dried for 24 hours at roomtemperature to evaporate the ethanol. After the treatment the yarnscontained about 25 wt. % phenolic resin.

In exemplary procedures, glass mat sheets with areal weight of 0.09g/cm² and thickness of 1.5 mm were impregnated with 35 wt. % ethanolphenolic resin solution and dried for 24 hours at room temperature toevaporate the ethanol. Expandable polymeric hollow microspheres Expancel551 DE were poured into a closed mold and filled (20 mm thick) the wholevolume of the closed mold. The mold was then heated at about 130° C. for15 min and microspheres were expanded and fused together, forming flatfoam core. The microsphere based foam core (with thickness 20 mm) wasplaced between two glass sheets containing uncured phenolic resin. Theglass skins were stitched to the foam core using the yarn containinguncured phenolic resin with a 2 mm needle. The distance between thestitches was 3.6 mm. The yarns were arranged perpendicular to faces.FIG. 4 demonstrates a general scheme of the invented sandwich panelafter stitching, but before filling with expandable microspheres

Example 6 Sandwich Panel Preparation: Infiltrating Microspheres into ThePunched Hole

In exemplary procedures, the needle punched holes within foam core werefilled with expandable polymeric microspheres Expancel 551 DE usingvibration tool.

In exemplary procedures, the assembly and microspheres were thereafterheated in the closed mold. The microspheres in the punched holes wereheated to temperature of about 130° C. for 15 minutes, and were shown toexpand and become fused together, forming light density foam core withinpunched holes.

In additional exemplary procedures, the assembly was thereafter heatedto 80° C. for 24 hours to fully curing of phenolic resin. Thin 1 mmlayer of fast curing epoxy glue Propoxy 20 was then placed onto surfacesof faces to thereby fix fiber beams to the surfaces of faces.

FIG. 5 shows a general schema illustrating the invented sandwich panelafter stitching and after filling with expandable microspheres

FIGS. 6 and 7 show general schemes illustrating the fragment of core ofcurrently known (FIG. 6) and the invented (FIG. 7) stitched sandwichpanels.

Example 7 Physical Caracterization Of The Sandwich Panel

Density:

The density of core foam and foam within needle punched channels was0.025 g/cc. The density of carbon fiber beams samples was evaluated as0.075 g/cc. The overall density of sandwich core with carbon T300 beamswas 0.1 g/cc. The density of Kevlar fiber beams samples was evaluated as0.045 g/cc. The overall density of sandwich core with Kevlar 29 beamswas 0.7 g/cc.

Compression Test:

Table 1 below shows the flatwise compression test data of the samples ofthe sandwich panels obtained in Example 5 hereinabove comparing withsamples' properties currently known in the art and illustrates thecompression data of the samples. The samples of the invention were basedon Kevlar 29 and carbon T 300 yarns and had dimensions of 20 mm×20 mmand 20, and 10 mm thickness.

TABLE 1 Compression strength, Material MPa Reference Nomex honeycombwith density of 100 kg/m³ 7.7 [1] Nomex honeycomb with density of 70kg/m³ 4.5 [1] Balsa wood with density 100 kg/m³ 7.5 [1] Balsa wood withdensity 70 kg/m³ 4.5 [1] Aluminum 5052 honeycomb with density of 6.0 [2]100 kg/m³ Aluminum 5052 honeycomb with density of 3.3 [2] 70 kg/m³ Thesamples containing 3.4 vol. % carbon 6.7 [3] T300 yarn beams and thefoam surrounding the beams. The height of core samples 20 mm. Thedensity of the samples 100 kg/m³ The samples containing 2.6 vol. % 3.2[3] Kevlar 29 yarn beams and the foam surrounding the beams. The heightof core samples 20 mm. The density of the samples 70 kg/m³ The samplescontaining 3.4 vol. % carbon 9.0 [3] T300 yarn beams and the foamsurrounding the beams. The height of core samples 10 mm. The density ofthe samples 100 kg/m3 The samples containing 2.6 vol. % 4.0 [3] Kevlar29 yarn beams and the foam surrounding the beams. The height of coresamples 10 mm. The density of the samples 70 kg/m3In Table 1: References are: [1] Baltek, 2002, ibid.; [2] Plascore, 2013,ibid.; [3] The present invention.

The data of the Technion's work was calculated based on the data forT300 yarn beams and Kevlar 29 yarn beams, and correlation with NASA datareport for evaluation of stitched sandwich panels respectively.

Shear Test:

Table 2 below shows the shear properties of the samples as obtained inExample 5 hereinabove (based on perpendicular stitches to skins),comparing with the shear properties of samples currently known in theart. The samples of the current invention were based on Kevlar 29 andcarbon T 300 yarns and had dimensions 20 mm×20 mm and 10 mm thickness.

TABLE 2 Shear Shear strength, modulus, Material MPa MPa Reference Nomexhoneycomb with density of 2.3 66 [1] 100 kg/m³ Nomex honeycomb withdensity of 1.4 47 [1] 70 kg/m³ Balsa wood with density 2.2 100 [1] 100kg/m³ Balsa wood with density 1.3 85 [1] 70 kg/m³ Aluminum 5052honeycomb with 3.5 680 [2] density of 100 kg/m³ Aluminum 5052 honeycombwith 2.2 458 [2] density of 70 kg/m³ NASA's sample - stitching 2.4 [4]technology evaluation report: 0.76 vol. % Kevlar 29 yarn beams; coreheight 25.4 mm; overall density of core - 138 kg/m3 NASA's sample -stitching 0.9 [4] technology evaluation report: 0.76 vol. % Kevlar 29yarn beams; core height - 12.7 mm; overall density of core - 138 kg/m3NASA's sample - stitching 0.5 [4] technology evaluation report: 0.38vol. % Kevlar 29 yarn beams; core height - 25.4 mm; overall density ofcore - 91 kg/m3 NASA's sample - stitching 0.6 [4] technology evaluationreport: 0.38 vol. % Kevlar 29 yarn beams; core height - 12.7 mm; overalldensity of core - 91 kg/m3 The sample containing 3.4 vol. % 1.4 [3]carbon T300 yarn beams; core height - 10 mm; overall density of core 100kg/m3 The sample containing 2.6 vol. % 0.5 [3] Kevlar yarn beams; coreheight-10 mm; overall density of core 70 kg/m3References in Table 2: [1] Baltek Inc. Technical Info, 2002; [2]Plascore Inc. Technical Info, 2013; [3] The present invention; [4]Stanely et al., (NASA-CR 2001-21105, 2001)

The data of the Technion's work was calculated based on the data forT300 yarn beams and Kevlar 29 yarn beams, and correlation with NASA datareport for evaluation of stitched sandwich panels respectively.

NASA data from Stanely et al., (NASA-CR 2001-21105, 2001) in the tablewere re-calculated to evaluate the overall density of core and torepresent the data for the sandwich core. NASA core density wascalculated by extracting weight of the sandwich panel with height 12.7mm from the weight of the similar panel with height 25.4 mm. Thedifference was the weight of the sandwich core. Dividing the weight ofthe sandwich core on the volume of this sandwich core gave the densityof the sandwich core.

Flatwise Tensile Test:

Table 3 below shows the flatwise tensile properties of the samples asobtained in Example 5 hereinabove, comparing with the flatwise tensileproperties of samples currently known in the art.

TABLE 3 Flatwise tensile strength, Material MPa Reference Nomexhoneycomb with density of 100 kg/m³ 4.0 [1] Nomex honeycomb with densityof 70 kg/m³ 3.1 [1] Balsa wood with density 100 kg/m³ 7.9 [1] Balsa woodwith density 70 kg/m³ 4.8 [1] Aluminum 5052 honeycomb with density of8.5 [1] 128 kg/m³ Aluminum 5052 honeycomb with density of 9.4 [2] 192kg/m³ NASA's sample - stitching technology 10.3 [4] evaluation report:0.76 vol. % Kevlar 29 yarn beams; overall density of core - 138 kg/m3NASA's sample - stitching technology 5.4 [4] evaluation report: 0.38vol. % Kevlar 29 yarn beams; overall density of core - 91 kg/m3Predicted data for the sample containing 56 [3] 3.4 vol. % carbon T300yarn beams; overall density of core 100 kg/m3 Predicted data for thesample containing 35 [3] 2.6 vol. % Kevlar 29 yarn beams; overalldensity of core 70 kg/m3In Table 3 References are [1] Baltek 2002, ibid.; [2] Plascore 2013,ibid.; [3] The present invention; [4] Stanely, 2001, ibid.

The data of the Technion's work was calculated based on the data forT300 yarn beams and Kevlar 29 yarn beams, and correlation with NASA datareport for evaluation of stitched sandwich panels respectively.

NASA data from Stanely et al., (NASA-CR 2001-21105, 2001) in Table 3were re-calculated to evaluate the overall density of core and torepresent the data for the sandwich core. NASA core density wascalculated by extracting weight of the sandwich panel with height 12.7mm from the weight of the similar panel with height 25.4 mm. Thedifference was the weight of the sandwich core. Dividing the weight ofthe sandwich core on the volume of this sandwich core gave the obtainedthe density of the sandwich core.

Tensile strength of the invented samples was evaluated by the followingprocedure: quantity of carbon and Kevlar yarn beams per surface area wasmultiplied by tensile strength of the beams

The evaluations of the tensile properties of the sandwich panel sampleswere performed according to published data (e.g., Stanely et al.,NASA-CR 2001-21105, 2001.

Without being bound by any particular theory, it can be concluded, basedon Stanely et al., that the flatwise tensile properties of sandwichpanel reinforced with fiber beams aligned perpendicular to skins areregulated by tensile properties of composite fiber beams themselves, andthat the foam surrounding of fiber beams will not affect the tensileproperties of sandwich panel.

Without being bound by any particular theory, it is noted that Stanelyet al. teach that in case of an upper thread being located in the foamcore and bobbin thread being located on the surface of the skin, thebobbin thread is stronger than upper thread. The upper thread plays arole in this case as a vertical fiber beam and bobbin thread as a rigidanchor to beam. Since high strength yarns are broken at flatwise tensilepanel loading, the yarns within panel core should be stiffly fixed toskins to thereby provide the maximum contribution to the tensileproperties of sandwich panel.

Example 8 Sandwich Panel Comprising Non-woven Webbing

Preparation Procedure:

In exemplary procedures, carbon T 300 yarns were impregnated with 25 wt.% ethanol phenolic resin solution and were thereafter dried for 24 hoursat room temperature to evaporate the ethanol. After the treatment, thecarbon yarns contained about 25 wt. % phenolic resin.

In exemplary procedures, glass mat sheets with areal weight 0.09 g/cm²and thickness 1.5 mm were impregnated in a phenolic resin containingbath.

In exemplary procedures, non-woven webbing of Kevlar 29 filaments wasprepared at lab. Yarns from yarn bobbin were cut to 100 mm length andwere combed with metal wire brushes forming 3D cotton like fiber webbingfilaments structure. The webbing was placed in a closed aluminum moldhaving a rectangular configuration with width of 31 mm, thickness of 20mm and length of 138 mm. Hollow microspheres 551 DE Expancel were pouredin the closed mold to fill the space among webbing filaments withassistance of vibration mean. The volume of webbing was varied in rangeof 0.1-1.0 volume % of closed mold, to which the microspheres wasinserted for dry infiltration. The microspheres were next heated attemperature of about 130° C. for 15 min in the closed mold and becameexpanded and fused together, forming foam. The microsphere based foamcore (with thickness of 20 mm) was placed between the two glass sheetscontaining uncured phenolic resin. The surfaces of the core foam weretreated with a glass paper to expose edges of webbing of singlefilaments. Phenolic resin was thereafter placed onto the surfaces of thecore foam. Glass skins were then manually stitched to the foam coreusing the carbon yarn containing uncured phenolic resin and a 2 mmneedle. The distance between the stitches was 3.6 mm. It is noteworthythat the 100 mm stochastic fiber filaments were not pulled out from thefoam. The assembly was then placed into the closed mold. The needlepinched holes within foam core were infiltrated with the expandablepolymeric microspheres using vibration tool. The assembly andmicrospheres were heated to about 130° C. in the closed mold and after15 the microspheres in the pinched holes were expanded and fusedtogether forming light density foam within the pinched holes. Theassembly was further heated at 80° C. for 24 hours to fully cure thephenolic resin. Stitching yarns were aligned perpendicular to the glassskins. A thin layer (1 mm) of fast curing epoxy glue Propoxy 20 wasthereafter placed onto glass skins to thereby fix the yarn beams to theglass skins.

The density of microspheres based foam supporting beams and containingKevlar webbing was 0.025 g/cc. The Density of carbon fiber beams sampleswas evaluated as 0.075 g/cc. The overall density of sandwich core was0.1 g/cc.

Characterization

Compression Strength and Elasticity of Uniform Size Microspheres FoamCompression strength was evaluated according to procedure of ASTM D1621of American Society for Testing and Materials standard. The foam sampleswere of cylindrical shape with diameter about 22 mm and length about 22mm with different densities and were tested for their compressionstrength using LLOYD Instruments LF Plus testing machine with maximumload 500 N. The data for compression strength vs. density for some foamsamples are shown in the FIG. 8 which demonstrates compression strengthvs. density for foams with ordinary wide distribution of microspheressizes. Linear decrease of compression strength at decrease of density isobservable. Foam samples based on Expancel 093 DU 120 microspheresbecame friable in the range of 6-5 kg/m³.

This effect may be derived from the lack of fusibility of largest sizemicrospheres responsible to low density of foam. Compression modulus offoam cell is determined by E˜t⁴/1⁴ according to Ashby and Gibson [LornaJ. Gibson and Michael F. Ashby; Cellular solids Structure andproperties. 2^(nd) edition. Cambridge University Press], where E denotesmodulus, t−thickness of cells and 1 is the length of cell.

In the instant case, compression strength of foam is determined bystress at 10% of foam deformation according to standard ASTM D1621-10.Thus, large sized microspheres give a major contribution to thecompression performance of the foam as the weakest cell of the foam isconsidered in the above expression for elasticity of cell. Large sizedmicrospheres have ability to be fused to form a foam.

Without fused spots, large size microspheres may form friable foam thatis observable on a foam with density lower than 6 kg/m³. It means thatthe ability to expand even more and to fuse requires optimal large sizeof microspheres. Too large size microspheres lose the ability to expandmore, to generate internal gas pressure and to fuse each other.

Extreme large microspheres can be heat-compressed by an outside pressureso as to be forced to fuse each other. External pressure may begenerated outside the mold or inside the mold by inserting smaller sizemicrospheres having the ability to generate pressure that will allowfusing large size microspheres.

For example, in FIG. 8 a foam having density of 10 kg/m³ has acompression strength of 40 KPa. When taking out the fraction of smallmicrospheres from the precursor powder, the density of the foam can bereduced by X factor and reduce compression strength by Y factor. Yfactor will be significantly less than X factor. That is, a remarkablereduction of density of more uniform size/density foam may beaccompanied with a neglectable reduction of the mechanical performance.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

All publications, patents and patent applications mentioned in thisspecification are herein incorporated in their entirety by referenceinto the specification, to the same extent as if each individualpublication, patent or patent application was specifically andindividually indicated to be incorporated herein by reference. Inaddition, citation or identification of any reference in thisapplication shall not be construed as an admission that such referenceis available as prior art to the present invention. To the extent thatsection headings are used, they should not be construed as necessarilylimiting.

What is claimed is:
 1. A honeycomb comprising foam, said foam comprisesexpanded polymeric microspheres coated with a conductive additive, saidpolymeric microspheres being fused to each other in at least one portionthereof, said microspheres are limited to size variation of less than15%, wherein said foam is characterized by a density below 15 kg/m³,wherein said density is characterized as being a uniform density, saiduniform density being characterized as having at least 90% of said foamwith densities that vary within a range of less than 15%, wherein saidpolymeric microspheres at least partially fill either one or both sides,and part, or all cells of said honeycomb, wherein said microspheres arecoated with multiwall carbon nanotubes at said carbon nanotubes densityof 1.5 gr/cc to 2.5 gr/cc.
 2. The honeycomb of claim 1, wherein saidpolymeric microspheres are selected from the group consisting of:polyvinylchloride, polyacrylonitrile, polyvinylidene chloride,polyimide, and any combination and/or derivative, and/or copolymerthereof.
 3. The honeycomb of claim 1, wherein at least 85% of saidmicrospheres are further limited to length or diameter variation of 160to 240 micrometers.
 4. The honeycomb of claim 1, further comprisingreinforcing fiber filaments.
 5. The honeycomb of claim 4, wherein saidfiber filaments comprise aramid fiber filaments.
 6. The honeycomb ofclaim 1, wherein said honeycomb is fabricated from a compositioncomprising: a metal, steel, aluminum, titanium, aramid fiber paper,carbon fiber paper or a thermoplastic material.
 7. The honeycomb ofclaim 1, wherein said conductive additive comprises a material selectedfrom the group consisting of: carbon, carbon nanotubes, a conductivepolymer, conductive metal particle, a magnetic metal particle, metalalloys, ceramics, a composite material and any mixture thereof.
 8. Thehoneycomb of claim 7, wherein said composite material is characterizedas having a size of at least one dimension thereof that ranges fromabout 1 nm to 1000 nm.
 9. The honeycomb of claim 8, wherein said carbonis in the form selected from the group consisting of: pristine carbonnanotubes, functionalized carbon nanotubes, single walled carbonnanotubes, graphene, fullerene, carbon black, graphite, a carbon fiber,and any combination thereof.
 10. The honeycomb of claim 1, wherein saidcarbon nanotubes comprise graphene.